If I increase the master image size of the Avatars from the default of 100x85, to something larger such as 110x95, it ends up leaving what appears to be a square border sized as the default of 100x85. This of course makes the larger Avatar look strange.

I don't know if it's a border, but I can find no options about increasing the border size to match the new master image size (if that is what this really is).

I did look through the siggen docs, but found nothing relating to this to put me back on track.

Any ideas?

the images used to make the avatars at 100x85 are only 100x85. When you increase the sixe, there is no image to fit that size... so you have to make new ones.

The background is not dynamically resized to account for a larger or smaller master image size
So you need to make new images to match your new size

Version 0.2.3 will set the background transparent if the image is too small