Hi Marius,
SU has an OpenGL limitation to be able to render on-screen an image resolution only up to 1024x1024 (or if you turn "Use maximum texture size under Window y Preferences > OpenGL, up to 2048x2048) pixels. So if you stitch relatively large images together, be prepared for this quality loss while looking at it in SU.
Nevertheless the resulting image will actually have its original resolution so if you right click on it in the Material browser and "Export texture image", you should be able to see the image with a normal resolutio in your external image viewr/editor. I am not sure however what quality loss SU will cause in UV mapped and united textures however so check it out first. Maybe the OpenGL preferences setting (above) will already result in a bearable quality.