20 x 18 would be 38" but if the ceiling is a huge volume, you might go bigger. Depends on how low you want it to hang as well. You might tape out the two circles on the floor below where it will hang with painter's tape and get some of the feeling for the space it will cover - that could be important. You might not want to have furniture under it (unless you have a killer foyer-style table to go there) and if the larger one eats up floor space under itself, that might determine going for the smaller one to allow room for the furniture to be arranged.

thanks! going with the 38" is cheaper too :)