When grading websites on loading speed, one of the things that Google's PageSpeed and Yahoo's Y!Slow recommend is to specify image dimensions in the <img> tag. I've implemented this on my own site, but I think it would be a good idea for it to be included in the code.
In the Social Media Widget, you can pick various sizes, but those are saved in the "icon_size" variable so specifiying the dimensions in the php file would be pretty straightforward.
I added the following code to the block for each social media site:
width="<?php echo $icon_size;?>" height="<?php echo $icon_size;?>"
If you wish, I can send you the social-media-widget.php file that I edited so you can incorporate the changes in the official code base.