Here's the file for my current avatar:
The dimensions are 108 by 113. However, it seems that when uploaded via Gravatar, the dimensions are always 200 by 200 regardless of the original size. I only just now noticed this because this and last week's avatar were the first time I haven't used avatars of 200 by 200 in a long time.
Uploaded on Gravatar: 200 by 200
Uploaded Locally: 108 by 113 (actual size)