It's smaller (obviously). An ATX size board will not fit in a Micro ATX case, although I believe that a MicroATX board can be used in an ATX case.
Similarly, a micro-atx power supply is physically smaller & has reduced power output. You can use a standard atx power supply on a micro-atx system if you can squeeze the PSU in (which you can't).
You will have to find a micro-atx power supply of those sizes, which is likely expensive, or change your case to a standard ATX case & fit a relatively inexpensive PSU.
I'll leave this to someone else.
Voltage-Yes. Amps No. Higher "Wattage" rated PSU's can supply more amps. Voltages will be the same.
Power = Volts * Amps.
A higher wattage is better, within decent brands. Cheap brands will advertise higher wattages, but the components are cheap, and the fluctuations in voltage can kill sensitive pc components.
Yes. It's called a standard.
Stable voltages. Large fans. Lots of large size conductors to devices.
Excellent online reviews which actually check the voltage stability, rather than just saying "It has pretty coloured cables". A few SATA power connectors also wouldn't hurt.
My personal advice is to swap cases with a full atx case, unless it's a home entertainment unit.