Update it seems the save destroying error previously mentioned happens if you try and enter the largest size while being in an unexplored area. This was very consistent in a save where I had already moved myself into an unexplored area, then tried to reload the save and switch to the biggest size.
Also, I believe the reason my first save is bricked is because, despite the fact that I am currently in an explored area, I previously, in that save, triggered the glitch and entered an unexplored area but was able to walk out of it back into an explored one and continue playing. I forgot that I had done that when I wrote the last comment, so to correct myself, the save was made after I had glitched into an unexplored area, not before I encountered it. Since I glitched into an unexplored area previously in the game upon reloading the save, anytime I try to switch to the biggest size, I get the error message despite being currently in an explored area.