If you are looking for examples measurements and so forth check out
Instructables
Should be able to find a couple of useful ideas and techniques there.
I built something like a computer desk chassis for my HTPC but instead of a desk just a TV stand. As far as the sizes for motherboard etc goes, I just made sure it was big enough for everything and actually over-sized the "chassis compartment" as it looked aesthetically better on the overall look so dont worry about it too much as long as it is big enough.