I just tried PTGui for my MAC. This was previously a window's only program. This current version is VERY easy to use and it produces VERY nice results. It does't cost a lot either.
I've stitched some large images (500+mb) and it hasn't crashed yet. It's not super fast at this size but it eventually gets it done.
You can download a trial here:
If you don't know what PTgui is, it's software used to stitch several lower resolution image files into a larger higher resolution image - mostly used to create panoramas, but can be used to produce high resolution rectangular and/or square formats with lower resolution files (from digital cameras or scanned images). This allows you to fake LF quality with smaller formats.