The smaller the diameter of tire you get the less overall traction youre going to have because of the changes to gearing.
What kind of riding do you do? If its primarily flat track/MX style dirt riding, the 18s with a 14t front sprocket might be the ticket. If you are doing more woods style riding, Id recommend the 20s with a 14t front sprocket and a 41t rear.
Advice on tires, gearing, etc, is going to vary depending on who gives it to you.