So in computer case sizing is Standard-ATX is the largest?

Are server cases clified as Standard-ATX and if it were 4 years old or something would this stop me from putting in certain motherboards?. I picked up a case at a thrift store , it had lighted fans and a clear door panel, but its un marked, i bought it because of its size hoping to build a really nice computer. But im worried now as to figureing out if my motherboard will fit. Is it basically the bigger the computer means any motherboard will with and how about the back plate wth att eh Input/Outpur? is that standard as well?