Hey! Thanks a lot for purchasing MRMO-STITCH!
The suggested "safe" maximum size of 128x128 pixels can be modified within the geometry nodes, but as the process is based around instancing "real" geometry, it quickly becomes extremely taxing on the hardware, with a 1024x1024 canvas producing over a million stitch objects, taking up roughly 5GB of video RAM with 10 million tris/5 million faces :)
What resolution are you trying to use?