There are several options to do so. It depends on which kind of photo editing program you have, the one that opens when you try to view pictures (i.e. ACDSee). This program should have a function called resize (or something similar). There you should be able to bring your Avatar to the size required here).
If you do not have such a program you can download one for free here:
http://www.nonags.com/nonags/grview.html 