As for what size the nut is, why don't you just run down to the local hardware store and have them match it to a bolt of a known size.
As for getting the stripped part off, is it the nut or the stud that is stripped? If it's the nut, you can try turning it couter closkwise while pulling on it and hope thta you can work it off. If it's the stud that 's stripped, take a hack saw blade and work at cutting the nut off. All of these methods are assuming that you have limited shop resources and/or mechanical skills. If you have more of either, let me know, I have many better ideas to try.
:baseball_w00t: