I would say that a small wedding would be no more than 50 or so people, a medium wedding around 100–150 [give or take a little], and any more than 150 guests would be a big wedding.
However, in my opinion, the size of the wedding really depends on the type of people who are there. If you have 100 close friends and you invite them all to your wedding, and your relatives as well, then that would be a small wedding because you would only have your close friends and family. Then say you have only twenty close friends, and you invite them and all of your relatives, and then also invite co-workers, some of your not-quite-so-close friends, and maybe your neighbors, that would be a medium wedding, even though it might have less people at the wedding than the previously mentioned small wedding, because you have all of the closest friends and relatives that are on the general small guest list, but then you have invited some not-so-neccessary people to your wedding as well. The same type of thing applies for a large wedding.