If by small form factor you mean half-height PCI, I really only know consumer sound cards. I’ve never seen a half-height pro-card.
ASUS Xonar DX is half-height, but has serious problems with the ASIO drivers (input basically unusable).
ESI Juli@ is a nice pro-card, and not technically half-height, but it comes in two parts which connect together.
If you were to extend these headers, you’d be able to mount it at a 90 degree angle, and then it would probably fit in your small form factor case.
Or, you could just use an external audio interface.
That’s all the advice I can give without knowing more about what you’re trying to do. Feel free to elaborate 🙂.
///Leif