By the way, there is no size called "ATX", either. PS-II is the full-sized form factor often simply refered to as ATX.
PS-II, PS-III, SFX, TFX, all are ATX power supplies. If you wanted to go with a full sized power supply, all you'd have to do is make sure it fit.
Many Micro ATX cases used PS-III power supplies, some actually had room for the longer PS-II (full sized) unit. The sticking point here is usually whether the larger supply will clear the power connector on the back of your CD-ROM drive. All you can do is measure.
It would be preferable to use a full-sized power supply, but only if you have room for it.
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