I don't know much about carbon fiber repairs. Craig Calfee does though.

As for sizing... most forks now a days are 1-1/8th inch as opposed to 1 inch forks. That measures the diameter I believe. You need an appropriate sized frame and headset. Road frames generally take 700c wheels except for some exceptions. Some older bikes take 27" wheels. Some small frames (under 50cm) may take 650c wheels.

Sheldon Brown is a good source. Along with Park Tool.

If there's nothing wrong with the frame and it fits fine, I wouldn't replace it. I would get the fork checked though. You wouldn't want a fork failure to happen while riding.

Also what kind of bike do you have. Do you have any pictures. We could help you out more.

EDIT: I thought the fork was the one cracked, not the frame. Regardless I'd get it checked out before riding. The LBS may be able to help you out.