What is the recommeded or Default profile picture size for the Apple Support Communities Profiles?

The largest size it will ever be used at on these forums is 80 x 80, which is the size of the user's larger image circle.

I used the circle mask as a template in Photoshop so I could size and center my avatar to fit.

My avatar is roughly 100x100 pixels

Not really - it's been there since about 2006 or so, LOL. But, it could have been either 100x100 or, at most, 150x150. I happened to find a round button in one of my apps and just put a letter on it. If you come up with one that is not round, it will look like the old fashioned widescreen movies on a TV screen with too much empty space at the top and bottom or the sides. So, round will look better.

It used to be that there'd be a warning regarding the size with limitations shown, but I just went to my preferences and wanted to make another avatar and all it did was direct me to my computer to choose an image. Maybe they have perfected it to the point where it is reduced to size automatically.

I think I made mine pretty tiny because it will end up postage stamp size here.