According to Campagnolo the max is 26 teeth. It can however be done with most bikes if you fiddle around enough. On some bikes you need to move the hub way back in the drop-outs, on others you need to use a very short chain (often locking out the large/large combination that should never be used any how). My personal experience points to the fact that the shorter the chainstay the more difficult to get it to work well.