The size of the cannister has nothing to do with the static pressure. CO2 will be at the same pressure at the same temperature in a closed vessel regardless of size, assuming there's still some liquid in it. Once the liquid all boils off from use, the pressure will sharply drop as the remaining gas is used.
Anyway, those CO2 cartridges are available in 8, 12 & 16 gram sizes. There's 450 grams in 1 lb. A 5 lb cylinder contains 2250 g. That's a little over 140 cartridges. 5 cases of 30 16g cartridges is around $225.00. You might as well get a 5 lb cylinder. It's cheaper, you could probably get the regulator, needle valve, and your choice of diffuser or reactor for close to the same and you won't have an anneurism changing cartridges a couple times a day.
Eheim Pimp #204.
58 gal planted Oceanic
Red Turquiose Discus, Japonica shrimp, assorted Blue Eyed Rainbows, Tetras, Hatchets, Danios, Rasboras
10 gal low tech planted Celestial Pearl Danio (Celestichthys margaritatus) & Emerald Dwarf Rasbora (Microrasbora erythromicron)