I personally prefer groups of GM +4 or +5, but everyone must see for him-/herself...

To your main question: There's no guide to joyful playing in any way. If or if not you are having fun will depend on many, many factors like

• do you know the rules sufficiently
• do you have an idea of the world you will be playing in
• do the players know each other
• does anyone in the group have experience with P&P
• where are you playing, and when
• what did you have for breakfast
• ...

obviously, I was kidding with the breakfast, though your physical and especially mental state is important.

the most important thing in my experience is that you do not stick to the rules if you do not like them. you may always proclaim so-called houe rules. the player's handbook, GM's handbook etc. are not the bible but merely guidelines...

And then it comes to your taste. If you are fond of detailed rules, I'd rather suggest Rolemaster as your system, if you tend to tell stories and don't like throwing dice, a World of Darkness system or a FATE system might be more appropriate (though this, too, depends on the players and on the GM: there are FATE GMs out there who make their players throw the dice more than any Rolemaster GM).

  I wouldn't have ordered a system out of the blue but tried it beforehand...

As a rule of thumb: If no player does have the feeling that he/she is forced to anything he/she doesn't want, and if the GM is able to produce some atmosphere fitting the setting, the experience for every participant should be good...

There are no hard and fast rules to making the game fun. Whether or not you are having fun will depend on many factors, like:

• do you know the rules sufficiently
• how familiar you are with the world you will be playing in
• do the players know each other
• does anyone in the group have experience with P&P roleplaying
• where are you playing, and when
• what you had for breakfast
• …

Obviously I was kidding about breakfast, but your physical and (especially) mental state is important.

The most important thing in my experience is that you do not stick to the rules if you do not like them. You may always proclaim so-called "house rules" to make the game work more to your group's liking. The player's handbook, GM's handbook, etc. are not the bible, but merely guidelines.

And then it comes to your taste. If you are fond of detailed rules, I'd sooner suggest Rolemaster as your system; if you tend to tell stories and don't like rolling dice, a World of Darkness system or a FATE system might be more appropriate. (Though this, too, depends on the players and on the GM: there are FATE GMs out there who have their players roll dice more than any Rolemaster GM.) I wouldn't have ordered a system out of the blue but tried it beforehand.

A rule of thumb: If no player feels like they're forced to do anything they don't want to, and if the GM is able to foster an atmosphere that suits the setting, then the experience for everyone should be good.

Aside, I personally prefer groups of 4–5 plus GM, but everyone has to find their own ideal group size.