When adding a new session in the profile manage, the dialogue apparently enforces limits. I have a three screen configuration of 3x 1200x1920, so an x-size of 3600 or even 3500 is perfectly reasonable to request, yet the GUI limis what I can enter to 3000.
Notably, the standalone x2go client does not have this problem, and sessions with screen sizes > 3000 work with no problem at all. I just cannot request them through the pyhoca-gui.
I'm running this on a Windows client but I would be surprised if this would not apply to all client o/s variants.