Look for the name of the installed mainboard and in the technical data for the form factor - mITX is quite small, E-ATX very large, the other specifications such as mATX and ATX in between. Please note that some office computers and servers have special formats such as SFF etc. - I cannot make any statements about this.
As the case accommodates up to E-ATX, any ATX-compliant mainboard and power supply unit should fit into the case. The case is large enough to fit larger (longer) graphics cards. So the chances are good that the components from your old PC will fit in.