Being a n00bie at this... What are the pro and cons of a smaller and larger Rim / Tire combo? faster, slower, harder on the bike?
The tire/wheel combo play a really big role in the handling of your quad. Basically:
smaller tires/bigger rims = less wheel roll, better handling on straight surfaces
bigger tires/smaller rims = more wheel roll, worse handling on straights but better in the rocks, it is also softer and better for jumping (in some cases)
bigger wheel/tire combo = higher gearing and higher top speed
smaller wheel/tire combo = lower gearing and lower top speed
Best thing to do for all around riding is find a fair balance in between. That is usually what the quad comes with stock.