reeferman- the max size is 150x150 pixels but there's also a file size limit (8.192k). There are two parts to resizing a file for internet posting- the first one you've already figured out and that's shrinking the dimensions of the image. The second part is compressing the file. In Irfanview, when you select 'save as' and choose jpeg as the file type, two boxes pop up- the one on the right has a slider and that's the compression side of things. Generally you can go down to a compression level of 60 with minimal quality loss so that's a good place to start. If the file size is too big then try 50 and so on.
Tinman- go to 'My RC' and select 'Edit Options'. Down at the bottom click on the Change Avatar button and a new page pops up-- scroll down past the premade avatars and click on the browse button to use a custom avatar that's on your own computer (from your own picture). Again, picture must be no bigger than 150x150 and 8.192k. For the caption over the avatar- go to 'Edit Profile' and about halfway down there's a Custom User Text field- that's where you change that.
Hope this helps,
Greg