I don't use GMS2, but should be about the same as GMS1.4.
First of all, I would like to mention that in those kinds of games, while expansion is possible, it has a certain limit - you can't just expand like crazy. Check out the top guys - if it is possible to expand, why aren't they expanding? Sure it costs money, you could say they're saving up for expansions, but then you can wait it out say for a year or two. They still aren't expanding (unless in that period of time the game updated and introduced more expansions - that is beyond what I'm talking about). That is because the game limits the expansions - the available buildings aren't meant to be sufficient for infinite expansion. Therefore they make expansions available slowly - ensuring there are buildings left to build (usually those games have a limit to how many buildings and what kind of buildings you can buy at a certain level - to buy more you have to level up).
However, if you want infinite rooms, I have to say,
theoretically a room is infinite in size. The room editor size is just to ease placing around objects and tiles, and setting the view. However, all this is achievable by code as well. Setting the view, placing objects and tiles. Moreover, it is more powerful than the room editor - for example, if you use the follow object option in the room view settings, it is limited to the room size. However, if you manually move it by code, it can go as far as you want. But with great power comes with great responsibility. Eventually when things get too big, you'll run out of memory, your game will crash, and it will ruin player experience. So I suggest making a chunk management system - that way you'll only load what you need.
Check out this video about infinite rooms. You can adapt it to suit your needs. Video:
TLDR: Infinite rooms are possible, but it means you'll have to do everything yourself. Also consider making a memory management system.
Hope my post wasn't too long, and hope I helped