Is mine too small?

i wouldnt expect it to hit 20, but it really depends on the gearing, the less teeth on it, the fastit goes, but the more chance the little engine will bog up and die

At least your expectations don't sound unreasonable.

Tire size doesn't matter when using a friction drive. The torque and top speed is dependant only on the diameter of the drive roller.
I've got a 25cc Homelite motor on my 1973 Schwinn Continental 10-speed. It's got skinny 27-inch, smooth tires, and rides in a "racing" position, so that helps bring my top speed a little over 20 mph. Drive roller is 1 1/8" diameter. But the old bike is heavy (and I'm about 210 lbs), so a lighter person on a modern mnt bike might do just as well.
I have to pedal when fighting the wind or going up hills; I don't mind. I get great mileage (and need the exercise).
A 1" roller would probably give you good results.
If you have perfect efficiency (no loss due to slippage), a 7/8" roller at 6000 rpm will give you 15 mph. These little motors can definitely handle 6 grand, but they have little torque and might top out below 6000 even on level ground with no wind.

The twenty five cc I built had great fun capacity. You can expect to pedal on hills. short steep ones, long gentle ones, but if you get good speed up going into the hill, it isn't bad at all. Even the short gentle ones can require some pedaling if you don't hit them fast. Most of the gentle ones the bike didn't slow down enough for me to catch up with the freewheel.

I now ride a 250 hub motor bike and it is very very much like the 25cc friction drive. The same pedaling characteristics. Maybe a little less top speed because it is a lower revving engine. But adequate for my needs exercise and fun.