Okay, so here's the problem. I'm going to be dming a large group of around 10 players that have mixed experience and since I'm used to being in small groups of around 4 players I'm not used to having this many.
Is there a campaign I can find which will support a large amount of players? Or is the best thing to do is just adjust modules to the number of players?
Don't. Split the party into two groups.
I've been gaming 25 years and have played in home groups, at cons, and at gaming clubs. Without any exception, in my experience games that large are terrible. You can try "keep people on track" techniques, but in the end you're putting lipstick on a pig. No one will be able to do satisfying roleplaying, encounters will be difficult to balance, social problems will emerge from the loud folks causing chaos and the wallflowers being overlooked more than normal, etc. Heck, even parking on the street and bathroom availability becomes an issue.
In our gaming group, whenever we get too many folks interested in one game, we just split it. We've had several times where one GM is running two parallel sessions of a given Adventure Path, for example. Or two people agree to run it.
Your players that want more fun per hour of time spent will just drop out anyway, either to not play or to set up a second game, in which case you get the same result but with the bonus of potentially some hard feelings.
Two five-person groups will get way more done than one ten-person group and have more fun doing it. If you can't spend 2 nights a week on GMing, then alternate them biweekly.
I prepared a grand campaign, and when I publicized it, I got many interested, initially 18 people wanted to play in the campaign, So I decided to split that into three groups of six. Since then (as with almost any game I have tried to organize) some players dropped out before we started, citing time-restrictions, distance problems, or plain just changing minds...
Regardless, I had prepared a grand campaign, and so I wanted them all to play in it. I split them into groups and had each group start in a different major city in my campaign. In this manner, I had a single grand campaign, with initially three groups (which dropped to two groups of four and five) all playing in the same world - even affecting each other.
I think that not only is it a good idea to split the large number of players into smaller groups, but that it is even better. There are many interesting possibilities when you have two adventuring parties wandering the same world making trouble and changing things....
The most brilliant part of my plan was that none of the groups knew of the others (this has since changed because it is hard to keep things secret now-a-days but it was cool while it lasted). In this way some quests and adventures that each group went on were sometimes influenced by other groups. The problem occurs when they are in the same place at the same time, and keeping the timing the same for the groups without screwing with the space-time continuum.
Anyway, split the groups.
I don't recall any modules that supported that many PCs. However, adjusting the encounters by doubling the number of monsters in each encounter could be potentially devastating, even with 10 PCs. You might want to try increasing the number of monsters in each encounter by about half, then increase the frequency of encounters.
Combat could slow to a crawl without some tools to help you speed it up. For example, I typically use an "initiative board" that is visible to the players, so they're aware of their upcoming turn without me having to tell them every time. It also helps that everyone is paying attention to what is going on, even if it's not their turn. This helps them formulate their strategy, and determine what they're doing, even before their turn.
Also, another trick I have my players do, is to have them roll all their dice at the same time for attack and damage. This actually helps a lot in my game, and may help in yours as well.
Lots of "split them into N groups" so far, but there is another option: The West Marches approach (which, as I understand it, is also similar to how Gygax ran his 20+-player campaigns in The Early Days).
In short, it comes down to not having fixed parties. One of the players says, "Hey, I want to head down to Elderone and deal with the ghoul king next Thursday. Who wants to come with?" and other players sign up to take part in that session. Then someone else says, "Those orcs we met at Close Inn are causing problems and I think they're spreading up towards Osgal's Mill. What say a few of us meet up on Monday to track them back to their lair and get rid of them once and for all?", so a different, but potentially overlapping, group of players meet up to do that.
And so on, with the basic idea being that the players divide themselves up into fluid, ad hoc parties based on what's going on in any given session rather than "Alice, Bob, and Charlie are A Party(tm) and they do everything together, every time."
Granted, this does require a campaign structure where you have a single home base that everyone goes back to at the end of each session to swap stories and plan their next expedition, but, if you have a campaign that's amenable to that and enough players that you're not comfortable running all of them at the same time, it's definitely an option worth considering.
This is not such a problem from the campaign point of view. Most campaigns could be played with more characters without much problems (apart from, maybe, the level of the combat encounters). We are also used to stories with big groups (think of the Fellowship of the Ring or the Heroes of the Lance).
However, this is a problems from the human point of view, or the Dungeon Master vs players point of view if you will. Most other answers have given hints about what to do as a DM, not about what campaign fits a big group. This makes sense, because a big group is a problem for the DM, not for the campaign.
I am going to suggest another way of dealing with big groups based on my experience.
The tradition of RPGs assumes that there is only one director, but this is not written in stone. I have played with two Dungeon Masters in the past with good results. I have co-directed groups between 5 and 12 players.
The benefits of playing with two Dungeon Masters are many, including:
It takes a bit of adjusting to each other the first time you try it, but if you have a good personal relationship with the other DM (which I assume is always the case) you will quickly find a good balance.
In DnD, the challenge of the combat encounters is an important fun factor for most groups. DnD 3.* assumes that most parties have 4-5 members. If your group is twice as big, your should enhance your enemies accordingly so that they prove challenging enough.
The challenges you face are primarily logistical - keeping everyone involved and interested in what's going on at the table, making sure everyone hears what everyone else is saying, and generally keeping it from devolving into mayhem. This may involve imposing a little more order on your group than normal. There absolutely needs to be a leader for the players whose job it is to report party decisions to the DM, basically doing as much crowd control as possible so the DM can focus on the game. That player doesn't need to also be the leader of the PC party, and in some sense it's probably for the best if they aren't, but for various reasons they usually end up being the same person.
No rulebooks at the table. If anyone forgets how their power works, they can't use it that session. Them's the breaks. Likewise, if there's a rule disagreement, DM fiat rules pending discussion outside of gaming time.
When rolling to hit, roll damage dice at the time. If you miss, ignore them; if you hit, you saved a few seconds to gather up dice and roll them. In a large group those 2-3 seconds per player can add up.
Depending on your players and their characters, you may be able to cut out a lot of unwieldy combat rules: Instead of per-combatant initiative, have the leader do a single opposed initiative roll for the group at the beginning and take turns based on how people are sitting at the table. Even better if you handle melee all at once and have everyone that does damage report how much and to whom. This can kind of bone high-dex strikers and characters that are especially hard to surprise, so don't force it on unwilling players, but it makes gameplay much faster.
