That small increase really wouldn't add too much to the forum bandwidth would it?
And a 100 x 100 size is just big enough to see much more detail in the avatars. 80 x 80 loses a lot of detail.
For example, my avatar is 80 x 76 and 5.52KB.
Here is my avatar at 100 x 95 and 9.07KB:

There is a big difference for a small size increase.
Most forums I belong to have a 100 x 100 10KB limit so I think it is a reasonable request.
And of course if people wanted to keep their 80 x 80s they could.
Thanks again Thomas for your work on the forum! (Just a little buttering up
