A 33t is the smallest that will fit on 110 BCD cranks. But some cranks' arms are long enough that the chain sideplates will hit the crank. You won't want to use the 33t on those cranks. The only way to find out is to try it. From my experience its more likely to be a problem with carbon crank spiders than with aluminum ones. I bought a 33t ring for a climbing race and discovered that my crank was one of the ones that it won't work on, so I have not tried the shifting. It'll be worse than with a 34t but I don't know how much worse.

You can get 33t rings from Harris Cyclery.