Use any image editor to do it. This can be something on your computer like MS Paint, or Photoshop, or GIMP, etc., or you can use an online editor like :http://www.online-image-editor.com/http://pixlr.com/express/
You'd have to resize it to a width of 150 or less and a height of 200 or less.
The problem with teaching you "how to resize and change your avatar" is that the program you use entirely changes the exact instructions for the process. It's highly dependent on the program UI. However, the main process is still what I outlined above:
1. Open the program.
2. Open the file.
3. Resize the image (or image canvas) to width of 150px or less, and height of 200px or less.
4. Save the image.
5. Go to this page to change your avatar: http://elliquiy.com/forums/index.php?action=profile;area=forumprofile
6. Hit change profile.