Is there a specific size that a smilie has to be to match the others? I don't see any specification when I upload, and so every time I upload a new smilie, it's a different size than the others. 

Any tips for adding multiple additional smilies/emoticions? Is there another plugin that will us to add more than just the handful that are already installed instead of having to upload each one individually?

The small, default set we supply as examples are 15 x 15 pixels. No reason not to totally replace them of course.

If you want to add many at a time then you could always just collect them together and copy them via ftp up to your server into the right smileys location. They will then appear in the list and enable you to fill the details in for each one.

Great, thanks!

Hope it goes well.