First an aside before I address the core of your question: Follow your gut instinct and severely restrict what your players can choose for characters. I'd strongly suggest PHB-only for two reasons: your inexperienced players will be able to absorb that much information eventually, and your experienced players can't break it badly at all. You don't want to overwhelm your new players and you don't want your experienced players either leaving them in the dust or end up effectively taking over other players' character-development choices in the name of "optimisation".
For your core question, I'm going to suggest a radical proposition: don't change anything. (Yet.)
Why? Again, two reasons. First, because anticipating the exact difficulty ahead of time is actually really hard. The CR system is already an approximation, but it's far from perfect. In fact, the degree of error that the CR system inherently has is going to be larger than the disparity introduced by increasing the party size by two. So given that it's an imperfect system, and given you're a new DM, you're unlikely to actually fix anything ahead of time. Which leads into reason two to leave it alone…
As you play, you'll accumulate DMing experience and direct evidence for what sort of things make encounters easy and hard for your group. The CR system is an idealised abstraction that can't possibly account for the particular strengths and weaknesses of your player group and their characters. As you gain more confidence as a DM, you'll learn how to tweak encounters to make them challenging and fun. Once you've got that experience, you can tweak encounters way more effectively than you can now.
And, as a bonus reason to not worry about it: The CR system was never meant to be a way to perfectly-balance your game. It was actually intended to be merely a way of estimating the difficulty of an encounter in general without the expectation that fights would be "even" with the PC party. You are entirely correct and safe to send your players' PCs up against easy fights, and up against hard fights. If you tailor your encounters too closely to the "level appropriate" CR then you encourage your players to turn off their brains and not consider whether a fight is a good idea or whether they need to get creative – and a creative mental game is certainly not supposed to turn off the players' brains, right?
So, to sum up this answer: Your best system for adjusting CRs is your own experience, and you need to give yourself time DMing the game to get that. In the meantime, you absolutely will not break the game or run a sucky game by having theoretically-underpowered encounters. Your players will still have fun, even if there are only 8 orcs in that room instead of 11.5 orcs.