Is it possible for a gnome Wizard to use a quarterstaff due to his size( In P&P, small size creature cannot wear a weapon two size category larger, gnome is small size and quarterstaff is large size).
I'm planning to play with an gnome illusionist.

Yes, I think he can.

Staves usually aren't that heavy and it's no problem to shorten them so the gnome should be able to use quarterstaff.

Can I have I true answer not a supposition please 'cause in 3rd Edition Pen & Paper rules they can't use a Quaterstaff. Did somebody try it in Icewind Dale II. I don't know if they have implemented this rule in IWD2. Rules from the Player's Handbook : "If the weapon's size is one step larger than your own (such as human using a greataxe), then the weapon is two-handed for you.... If the weapon's size category is two or more steps larger than your own (such as a gnome tying to use a greatsword), the weapon is too large for you to use."

If we knew the true answer, we'd give it to you. I'm pretty sure that your gnome can use the staff.

Since all characters start with staves at the very beginning of the game, I would say yes.

My Gray Dwarf had one....didn't have a gnome though.

Javalar, Dwarves are medium sized characters :p

I had a halfling in my party, (rogue) and he started with a quarter staff just like everyone else, and stuck with it for about half the prolouge.