A good sign of a good power supply is the weight; the more the better, thus in many cases making the size larger. The weight is directly related to the quantity, quality, and size of the material used to build it.

Here are some dimensions about power supplies for different forms (ATX, Mini-ATX) that I found through some research that may help:

The following information was obtained from: http://www.directron.com/psu.html

ATX: 6x3.5x5.5", HxWxD. Most common. Uses 4 mounting screws.
Mini-ATX: 5x3.5x5", HxWxD. Rare size. Uses 4 mounting screws. Can be used in a regular ATX case, but often not the other way around.
MicroATX: 5x3x4", HxWxD. Use 3 mounting screws. Not interchangeable with ATX or miniATX.
Flex ATX: Even smaller than Micro ATX. Various sizes according to case specs; often not interchangeable.

so basicly no?

The answer to your question about same size power supplies is - different forms (ATX, Mini-ATX, etc) are different sizes, but as I understand it, each form as a set dimension standard for that form. Hence the listing of different sizes for each form ;-)

The easiest way to resolve the issue would be to take measurements of the power supply, or read them if available on the power supply itself, and simply find a new power Micro-ATX power supply with the same dimensions. Or buy a new case with a power supply. Full ATX will probably be better for your needs if you are considering any serious upgrades.

Or, you could go to a site like Pricewatch or [url=http://www.froogle.com]Froogle[/b] and type in "MicroATX". They should come up with some responses, I'd imagine.

Not all micro atx cases take smaller dimension power supplies,but i do believe your does .you would be better off buying a full size atx case with power supply,and transfering all you goodies into it !

Lol no way a Full ATX case is way too big for me anyways I contacted eMachines and they said that even a 400watt powersupply should fit in my case.

and with the microATX PS the best one i can find is 250watts and thats what I have now, and is 50watts a big difference or should I go even bigger?

You haven't said why you want to go to a larger power supply, a 300 or 350 should be more than adequate for your power needs. If you need any more that I would be worried about ventlation and go to a larger case.
Good luck which ever way you go
Butch

well if you don't need the bigger and better{runs cooler } case, then you probably don't need the extra power !