By definition, as laid out in ATX Form Factor Specification v 2.2, ATX motherboards are 12.000” x 9.600”. There are similar specifications laid out for related form factors such as Mini-ATX, MicroATX, FlexATX, and others, but those do not concern you.
All you need to know is that all ATX mobos are 12“x9.6”, any case that is listed as ATX conforms to ATX standards and thus will accept any ATX Motherboard (though it may be too small to allow certain add-on cards, such as your larger video cards), and that the ATX form factor is the most widely used; most mobos and cases ont eh market now are ATX.
I share @Fred931‘s curiosity as to what this build is for, since AM2 is deprecated five years ago.
@mrentropy Your experience should tell you how ubiquitous ATX is; you have to look for stuff that is not ATX. And yes, ATX Spec v2.2 also specifies screw locations.