I've not noticed any issues with maximum sizes however I did just download a GFX 100S sample image, and you're correct, I can't get it to enlarge it to enlarge it by more than 2.75x either. In playing with it a little, it seems that it caps the image size to 32,000 x 24,000 pixels, which seems like it kind of defeats the purpose because it is easy to stitch together a panorama bigger than that, and to do so without paying for additional software.

I contacted Topaz but as it is the weekend, I suspect I won't hear back until Monday. I was able to enlarge a smaller (12MP) image 6x, however. Specifically, when I select any size larger than 2.75x, it only gives chooses 2.75x as the enlargement. However, Gigapixel simply will enlarge the 11,648 x 8,73 pixels images from my GFX 100s more than 2.75x. I was surprised, because Topaz' website says they can be enlarged up to 6x without any loss of quality. I purchased Topaz Gigapixel AI today and it seems that the largest it allows me to enlarge most of my full-size image files is 2.75x.

Topaz labs ai upscale