Updated for Stellaris v2.7.2 and the Federations expansion.
Notes: I have been personally unable to load any galaxy sizes at 3k stars and larger.
I don't believe it is an issue with the mod since I've made no major alterations from previous releases, and is possibly either an issue with my under-powered CPU, or something in the game was changed that is causing this that I have yet to identify.
Please comment on this file, with your hardware info (GFX card, CPU, and Ram) and what game starts you tried, if you have any issues starting a new game with any of the custom starts.
I was able to get the game to load at 10k stars today, but it took me at least 30 min, after which I was out for a while, allowing it to continue to load. This reinforces my thought that my processor is to blame for the issues with starting a game with custom galaxy sizes.