I can help!
Firstly wat current tire size do you have? for example are you looking at 26 by 1.95?
Wat type of bike have you got, is it a mountain bike, road bike or hybrid?
If you go for a bigger tyre size than you have now, you will get better grip on the road, but less maximum speed.Also you need to think about the conditions your going to be riding in?Slick tyres in the wet, ouch!