I like the 120x120 rule here. Big enough you can actually make out what it is you are seeing but small enough it isn't going to throw off the page size. My file size between 10Kb and 12Kb
If you use avatars this means that when somebody browses a topic in a forum on your board, the avatar images also have to be loaded. This may increase the page loading time, although your board has to be really big for this to be of some concern. so you have to decide what's best for your board and users. What you can do, however, after you enable the use of avatars, is to change the dimensions and maximum size of avatars that people can upload.
If this is of concern to you and you have many users, you can reduce the maximum avatar file size.
(also depends on your hosting package)
Last edited by Cryo
on Sun Nov 09, 2014 4:43 am, edited 2 times in total.