Thanks again Daedalus, your change did the trick! I did quite a bit of experimenting last night and have a few observations.
Even though I was able to extend the maximum size of cubemaps, they are currently restricted to being square. I made some changes to allow non-square cubemaps but kept having problems… I imagine other parts of the code are assuming square dimensions. Ideally a cubemap used as skybox would have an aspect ratio of 2:1.
I was unable to get anything higher than 2048x2048 to import. The editor silently crashes after a long time.
Makes sense, though in this situation I am porting a game that uses a skybox that would be difficult to generate procedurally. The game uses six 1024x1024 images to form the skybox, giving it effectively 4096 horizontal resolution. The ideal cubemap resolution would be 4096x2048 which isn’t possible at the moment. Due to these limitations, for now I think I will just hack up a traditional six-sided skybox.