Guys, i'm thinking about to keep only 5 members in my party so that I can reach highest levels quickly (it should be 3 warriors, 1 priest and 1 wizard). But there are two problems with this strategy:

1 - If I normally reach the highest level even with 6 in my party, so, It is a stuppid thing to try to play with only 5 (how to know it if I don't know How XP is available in the game ?)
2 - Maybe the amount of XP that I'll get with 5 members will reach the level cap before the ending of the game, and the game could become boring.

Thanks a lot : )

You guys who has experience in this game could give me a help: is it a good thing to play with 5 or may I keep 6 in my party?

I PERSONALL LIKE 6 GUYS, 2 FIGHTERS, 1 MAGE, 1 CLERIC, 2 THEIVES, ITS A GOOD BALANCE THAT HAS GOTTEN ME TO LEVEL 7 AND I HAVE MAXED OUT MY LEVELS ALREADY.

I'd just go with six. There's plenty of EP in this game, so you don't have to worry about levels.
My party's usually fighter/mage, fighter, fighter/druid, mage, cleric, thief/illusionist (or me, Korgan, Jaheira, Edwin, Viconia, Jan).