I opted for a 10 pounder because the extra cost wasn't that much ($17) over a 5 pounder and because most of the charge of filling the tank is the set-up cost. Any shop in town charges me $2 more for 10 lbs than for 5 lbs.
If you force-carbonate, figure around 9-10 kegs dispensed from a 10 lb tank. Try welding supply shops in addition to the medical supply shops. They shouldn't have any problem filling 5 lb tanks.
If you're planning on putting the CO2 tank inside the fridge, make sure a 20 lb will fit before you buy one. They're pretty tall. My 10 lb fits easily and leaves more than enough room for four corny kegs.
So long, and thanks for all the beer...