This should be possible. I don't know how to add custom image sizes, but I can include one bigger size to choose from. There were two reasons why I did not include bigger sizes in the first place:
1. The image files are saved on my server and will be pretty big. I have to delete them manually, because I don't know how to code a script do delete the files after a set periot of time. This is not a big problem, but I will have to remember to delete them more often. If anybody can help me with this this would be awesome!
2. The bigger the image the more you notice the small offsets and drawing errors. When I started to work on the SP-Studio I drew everything with avatar sizes in mind, so when you zoom in the old drawings look really ugly. I already corrected the worst of them, but I still feel uncomfortable when I see some SP-Studio images in very big sizes.
But this is just a personal thing and no reason to keep you away from higher resolutions.
I wanted to do some small corrections on the image sizes anyway, so I will see if I can include a bigger size in one of the upcoming updates.