In the past, Asus "barebones" systems have used:
1) MicroATX power supplies. These can be hard to replace, depending
on the mounting hardware, cooling fan location and so on.
2) Some of the barebones motherboards used, were "L" shaped,
rather than being standard ATX or microATX motherboards. In
those cases, a standard motherboard may not fit.
The best way to check what you're getting is:
1) Download the user manual for the Asus computer, from support.asus.com ..
The user manual sometimes has one or two pictures of the interior
of the computer. Look for non-standard parts that way.
2) If the computer is for sale on Newegg.com , they may have some pictures
of the inside of the computer.
ASUS Essentio CM5570-AP002B (standard power supply, microATX motherboard)..
http://images17.newegg.com/is/image/newegg/83-220-036-Z06?$S640W$
The motherboard is not exactly 9.6"x9.6". It looks like it might be
9.6" tall, but perhaps 7" to 8" wide. To replace the motherboard, you'd
need a slightly narrow replacement microATX. Otherwise, the motherboard
connectors, may bump into an installed disk drive. So while it is
nominally standards compliant, you cannot put just any motherboard
in there. Similar to doing a home build, you have to examine the
motherboard you're about to purchase, for suitability for the computer
case it will be installed in. Sometimes, a motherboard has the main
power connector, in a particularly bad location.
http://www.newegg.com/Product/Product.aspx?Item=N82E16883220036
For more popular computers, you might find a review for them on a
web site somewhere. I don't know if the Asus Desktops are reviewed
by anyone or not. That would be another place to look for pictures.
Paul