How big is the wall plug?

Hello Michael:

The part of the plug that goes into the back of my G5 is 1 1/2" in diameter. That is an early G5 iMac. If I were you, I would wait until the computer is delivered. The newer iMacs may have different plug dimensions.

Barry

Why not just drill the hole small enough for the plug that goes into the wall? This is a standard size and you can use any appliance plug as a model. I would make the hole a little bigger just to allow the plug to slip through easily.

You can get plugs which are designed specifically for the purpose of going through table/countertops. They are adjustable and have a little cover to open/close allowing the plug ends thtough.
Check at Home Depot, Lowes, etc.

I have to pipe in here with unsolicited advice. I consider using either an extension cord or a power strip instead of feeding the CPU power cord through the hole. It makes it easier to unplug for whatever reason you may need to do so. Of course there esthetics to consider.

Best measure once you receive and then drill the next larger size hole - you may want to connect peripherals through the same hole so keep it roomy. Once you've decided on the hole size, you can buy a grommet to make things look nice.

Regards, SudsK

Once you've decided on the hole size, you can buy a grommet to make things look nice.
Here are a few examples. Check out Home Depot and such.

Cable Organizer.com