I have a different avatar that I would like to use, but it says it is too large.... How can you shrink it down?

You need to get image editing software. Most of it is pricey but there are some free utilities. This one looks like it could be good:

http://www.download.com/FastStone-Image-Viewer/3000-2192_4-10368265.html?tag=lst-0-16

Of course anything you download make sure it does not contain spyware (as a lot of freeware does these days). Reading the reviews of this one it looks like you're fine.