So I tested the FOUC with a smaller avatar, and here are the results:
The avatar shows up in pixel-perfect scale at whatever resolution it was uploaded.
If I upload a full (1Kpx) ^2^ avatar (the maximum size allowed), I can expect to see my huge face every time I click a local link.
I suppose the question could be:
What is a good size for an avatar? What is the largest box that an avatar should fill?
These avatars in forum posts sure don't look the full (1Kpx) ^2^. I imagine it would be easier on everyone's hardware if the avatars were sampled at a scale that is reasonable for the site.
Alternatively, you could just _suggest_ that we use a reasonable avatar size (we can always link the full size image in our profile) by suggesting the largest size you display during any normal use, and put that suggestion somewhere near the upload functions.