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http://www.formfactors.org/developer\specs\atx2_2.pdf
'atx spec' in google
Don't know if it gives actual size, but you can always take the info and draw one of actual size and then print it out. Visio, or cad, both expensive. Or you could take an existing mb tray and trace out the mounting holes on a piece of trace paper, then use it to mark and drill fresh metal.
Why dont you measure a mobo?
Well... you could use the actual motherboard. plop it down and mark through the existing mounting holes. it would be a custom fit. Would be an issue to make sure it was in the right position though, assuming you want it to be in a case and have the io panel set properly.
edit: oh yeah, i guess thats what ring.of.steel was getting at.
When did reading a spec and using a ruler become difficult?