http://www.jql.co.uk/avatarsizer.htm
Haven't tried this program but it came up first in a google.com search. It also says claims to be free.
If this doesn't work, you can always try to use Window's paint program. You would have to copy and paste the image into your paint screen, edit > select all, move mouse to lower right corner of the selected area, (mouse will make a double arrow letting you know its hovering over the right spot), then left click and hold, drag mouse diagonally up and left, until its the size you want. Do the same to the entire image. Save your image.
(Formats you can save in, like JPG is much smaller than BMP but less quality.)