The size depends on the min/max temperature and the volume of water in the system and has nothing to do with the BTU rating. Heating x amount of water y degrees means it will expand z, and the z is what your expansion tank must allow. A bigger one won't hurt, but one too small will either wear out fast or allow the pressure to rise once it gets full.
www.watts.com has a calculator on their site to determine the size you need (you could use the volume they come up with and then compare it to the size of other brands). You need to estimate how many gallons of water are in your whole heating system, though. If you take the length of the piping and radiators and add 2-10 gallons for the boiler, depending on the type, you should have a good WAG. the smaller one may be fine for your application, but you need to run the calcs first.