When I first wanted to keg (in '96), I bought 3 kegs that would fit the best in my basement fridge and be portable - 3 gallon kegs. The were MUCH more available and cheaper then. I have 3 of them. I usually do 5.5-6.5 gal batches. Actually, I do 11-12 gallons split between carboys, with a different yeast in each. I keg 3 gals and bottle the rest. Works great. sure wish I could find more 3-gallon carboys cheap (I know, - don't go there).
Plus, I find a difference in bottle-conditioned vs. keg conditioned (force carbed) taste. It's almost like having 4 beers out of one 11 gallon brewday!
I force carb my kegs nowadays, but originally, I was told that you used a bit less priming sugar in the keg than when bottling. Someone please enlighten me.