When you choose Medium / Large or whatever size when starting a new game, the game doesnt actually generate any maps, it just picks a random Seed_25971 named map.
You can go into the cartographer, and make any map you want (or generate one with your new map scrips, which i'll try now!), then save it as Seed_25927 or whatever, look at the original files and mimic their names but with a unique number.
Then that new map will be in the pool to have a chance of being picked when starting a new game (or you could just load it manually, ofc).