It depends on the substrate you use. The small 10-20 zoo med reptitherm may be on the small side, but it should heat to at least 90 degrees. But ideally you want something that'll cover at least 1/3 of the tank, so the medium one may work better. They're cheaper to purchase from amazon than retail stores if that helps.
I have the small 10-20 in my 20L and while it does provide the correct temps right over the pad, it does not heat enough of the tank. If I was doing it all over, I would go for the medium.