I have 2, a 15# and a 20#. (same diameter, just taller) I bought the 20 first but found it to be big in the back. It's extra height would not let my rear seat back go all the way back and latch and it also kept me from running my duster when I wanted to keep crap out of the back. The 15# is just enough shorter that it solves both problems for me.
I try not to do major air ups with my CO2 tanks but rather just give me enough boost to be road worthy (raise from 5psi to 10-12) or to air up the tire that burped a little. Airing up and down all weekend needs to be done by an air compresser.
If it were me doing it all over again knowing I am running 38+ tires, I would not get anything smaller than a 15#. If you have the extra room in the back and the taller tank won't bother you, go 20#.
DISCLAIMER; I really don't remember the sizes but I looked at my tanks this morning. One is stamped 18.6 and the other 14.(something). That's what I am basing my sizes on.
They are both Ultimate Air tanks so maybe Brian can shed some light.