Depends on the CA. Microsoft CA's are web-based and only require TCP port 80. Check with your specific vendor's documentation to see what you need to allow through.
If you're still not sure, connect to it from an inside PC and try and see what ports it uses. If it's all web-based then TCP port 80 at least, but see if it maps to another port or something after the initial connection. A sniffer might help to. Once you have an idea of what ports, allow them through in the PIX then try a connection from the outside. If it doesn't work, enable syslog on the PIX and look for "deny" messages, they'll tell you what protocol and ports the PIX is denying through.
