You could try the Soothies brand bottles. Soothies are the same as the pacifiers they give the babies at the hospital; they have the green nipples. My sister in law's baby wouldn't take any bottles either at first for formula or expressed breast milk, until she tried Soothies. Also, it's not necessarily a smaller nipple but the flow from the nipple. I had the same problem with my son. We use Avent and Playtex VentAire bottles but with them being so little you have to make sure you have the 1 nipple, for slower flow.
I hope that helps. (From someone who had problems with bottles, at firts, too!)
P.S. If you try bottle after bottle and different nipples and still have problems, as another person said, it could be the formula. I breast fed my son for about 2 1/2 months and he took that fine, but could not take regular formula and I had to switch to soy. MUCH better, less problems at feeding time, and less spit up later.