I'd just get the biggest size you can comfortably carry.
I have a bunch of carriers, but yeah most of the ones made for cats are too small for my Maine Coon unless she curls up...
I bought a pet stroller when Roxy my Maine Coon and Church my DSH were kittens because it was getting too heavy to try carrying them both places (vet, pet store, cat shows, etc...) They ended up outgrowing it so I got a bigger one, then a bigger one... It is nice to have because I can just wheel it and I don't have to lug several heavy cats around!
Here it is with two of my younger cats in it:
http://www.chicagocanine.com/photos/...tdogstroll.jpg