You want the medium size, not the small one. I was trying to find measurments on the carriers. Figure the kitten will be about 10 lbs when grown. Better to have the carrier a little bigger then to have a small one and the kitten outgrow it.
What are you planning to use it for? Just vet visits? If so, i'd say it should be ok.
If you have not already bought one though- i might suggest a little larger of a carrier - that way if there is ever an emergency- you will be able to fit a small bed and litter pan in there and keep your cat confined comfortably during a possible emergency/etc. It's good to have a safety plan.