Hi,
We'd need to know the exact case (or just look at the PSU) as there are two types of PSU's for SFF's in general:
a) ATX, and
b) SFX
You should also use semi-modular or fully-modular to avoid wire clutter.
Check PCPARTPICKER once you figure it out. Depending on the CPU, OC etc probably 450W+ is fine.
I'll try to link a PSU but I don't know if SFX is what you need.