I can't seem to find the engine requirements for the different pylon classes. We would like to race the smallest class there is. Thanks.

The best thing to do is find out what is being run in your area and then see if it peaks your interest. For the most part the physically smallest airplanes would be the Electric formula 1. But starting out in Q500 would be the easiest way to go.

The heck with official racing classes! Race what you want with your friends. We race with Cox .049 Sure Start engines. The only rule is the engine must be a reed valve Cox motor. Anything else goes!

AMA event #423. 1/2 A. The rules are here. http://www.modelaircraft.org/events/compreg.aspx

Look at this http://www.matneymodels.com/vixen.html
Speed 400 on a brushed SP400 and 2 cells dose about 75mph

If you are looking for the smallest pylon racer and not necessarily a "class" consider the Tiny Toni. It was published in the AMA Model Aviation Magazine. You can print out the full size plans on legal size paper. The weight with a 500 mah 3S lipo battery can be kept under 250 grams (mine was 238 grams), so no remote ID module or registration is required. I radars at 68 mph. Check it out at:
https://www.modelaviation.com/tiny-toni