I see no problem with the avatar "rules" at all.

If you want help or need a better program, there are plenty of free programs out there way better than MSPaint. I know one is called GIMP if you wanted to give that a try

With paint, though, if you "select all" (or CTRL+A), then you can resize the image itself. You may need to resize the canvas afterwards to the resized image, but yes, it can be done to a specific pixel size rather than resizing by percentage.