Found this:
What size should I choose?
* 26T (64BCD) Oval chainring has the ovality of 24/28T and is best for someone who uses currently 26T round ring.
* 28T (64BCD) Oval chainring has the ovality of 26/30T and is best for someone who uses currently 27 or 28T round ring.
* 32T oval ring will suit best someone who uses 30 or 32T chainring currently. While pedaling, you will feel quite similar effort to the one riding round 31T ring (as the smallest diameter of the oval is 30T), but you will gain the speed similar to riding 33/34T chainring.
* 34T Oval chainring has the ovality of 32/36T and is best for someone who uses currently 33 or 34T round ring.
* 36T Oval chainring has the ovality of 34/38T and is best for someone who uses currently 36 or 38T round ring.
from: http://absoluteblack.cc/oval-104bcd-chainring.html