Okay, I was bored, so I decided to calculate this. If I remember my high school chemistry right (grams to moles, moles to grams, etc.) I can figure out how much CO2 should theoretically be generated by each suppository. Here goes:

According to the box, each one contains 0.9 g potassium bitartrate (mol. weight 188 g/mol, Wikipedia) , 0.6 g sodium bicarbonate (mol. weight 84 g/mol). That is equal to 0.00478 moles of bitartrate and 0.00714 moles bicarbonate. This makes the bitartrate the limiting reagent. I think the number of moles of CO2 produced should be the same as the number of moles of bitartrate, so we have 0.00478 moles of CO2 being produced. The ideal gas law is PV=nRT, or V=nRT/P, where n is the number of moles, R (=0.08206 L atm / K mol) is the universal gas constant, T is temperature in Kelvins, and P is pressure. Assuming that the rectum is 38C or 311 K, and that gas pressure isn't too much bigger than atmospheric pressure, this gives

V = (0.00478 mol) * (0.08206 L atm / K mol) * (311 K) / (1 atm) = 0.122 L

or 122 milliliters.

Someone (a chemist around here, maybe?) check my math. I don't know about you, but my enemas are usually a lot bigger than that. No wonder they don't work.