Since you're riding a mod you don't need a huge rotor or expensive brake to make it work well. If it's for the front wheel, get an Avid BB5 180mm and an Avid SD7 brake lever. It's cheap and it's amazing. Don't buy the BB7, it's not worth the extra money and the braking performance isn't any better. You could use a 160mm rotor but if you're after superior braking performance, a 180 will be the better choice.