Changing the thumbnail size parameters doesn't do anything for existing images. They have to be reuploaded to be resized with your new settings. If you want a consistent size regardless of how large or small an image is you'll need to ensure every avatar was uploaded with the correct settings or use CSS to force a size you want.

Using CSS to force image sizing distorts the images. Changing the parameters also is not going to force all the images to resize as all that'd accomplish was crashing peoples servers. You'll need to adjust your parameters as necessary then re-upload or consider using custom CSS to force a sizing, which will distort them.