Definitely get the biggest one that will fit your application. I have a 5lb cylinder, because that's all that will fit in my kegerator, and I can carb and serve about 6-8 kegs off that tank.
I'm working on restoring a 1949 Crosley Fridge, and have my eye on a 12lb tank that will fit in there. I thought about getting a 20lb tank and keeping it outside the fridge, but cold CO2 performs more consistently than room temp CO2, so I'm just going to get the biggest one that will fit inside the kegorator.
Primary #1 - Midnight Ryeder (Midnight Wheat and Rye)
Primary #2 - Florida Weiss
Primary #3 - Kane-DOH APA (Honey Citra APA)
Secondary #1 - Downtown Flanders Brown (brewed August 2012)
Keg #1 - Raspberry Florida Weiss
Keg #2 - Cinnamon Raisin Cider
Keg #3 - NONE!
Bottled - NONE!