Well a good estimate for the size difference is that you multiply 15 x the cubic inch if it's a 4500 PSI tank. So say if you had a 68/4500, you would multiply 15 x 68, which is like 1000. The rule for a 3000 PSI tank is that you multiply 10 times the cubic inch. So if you have a 68/3000 tank you multiply 10 x 68. Which is 680. This is only an estimate, you could get upgrades for better efficieny like a QEV or something. If you live so far away from a fill station, I would either get a scuba tank, or just don't bother, until you start playing at a field with N2 fills.
If you have anymore questions pm me.
Paintball? Wtf is that?