Hi!
I never tried to do large SNES games (for now), but from what I've read the maximum "standard" size for games is 4MB (32 Mbits): https://problemkaputt.de/fullsnes.htm#snescartridges
But by using special chips inside the cartridge, you can go way larger: the two largest SNES games from the commercial era (Star Ocean and Tales of Phantasia) where 6MB (48 Mbits) each. And nowadays, using the SD2SNES / FXPAKPRO Flashcart with the MSU-1 mapper, you can go up to 4GB in size: http://nerdlypleasures.blogspot.com/2018/01/sd2snes-and-msu-1.html
But honestly, with the way the SNES stores audio and graphics data (very efficient formats = small sizes), 4MB is quite a large amount of space to make cool SNES games :)