Well, I think you basically found the maximum resolution. The use of signed 16 bit integers just cascades throughout the code, and the amount of Flash and RAM left is just humorous. Just to test adding a single 32-bit integer resulted in this cute bit of debug text:
Binary sketch size: 32,250 bytes (of a 32,256 byte maximum)
So the maximum resolution is theoretically 32,767 X/Y, but I’m sure we should reduce that in case there is code that relies on larger values (off screen stuff). So let’s say 32,000. Photoshop won’t even allow me to save a BMP that big (16×32000). If you can make one, I’ll test it out. It would be an even 2000 frame filmstrip.