I would recommend 1.85 for beginning and medium talent riders, with a psi around 100. Less than that and you run the risk of damaging your rims if you hit a curb. As you progress to the expert level, 1.75's with 140 psi will help you on some of the bar spin rolling stuff as you will be able to keep your speed up even with a lot of tire movement, because of the lower rolling resistance.