I play on a 1440p monitor, and so I usually play in fullscreen, but this caused some problems with my display capture blacking out for a moment when I switch to windowed to open another client (judgement hall/photoshop flowey sort of thing). I found a piece of software called Sizer.

I checked the rules, and since it doesn't read anything from the screen or game memory nor does it control the game with macros, it should be OK, but I want to be completely sure.

TL;DR: Would I be able to submit a run where the window is a different size than the game allows because of external software?

have you tried normally resizing your screen without an external software? you can fullscreen and undo it with f4 i believe

i don't use fullscreen, i just have the smaller window in the middle with my kps tracker and autosplit on either side so i can see them during runs. im sure it's fine it doesn't affect the recording/streaming quality in videos (in OBS at least)

side note: oops i just realized this is undertale not deltarune take everything i said with a grain of salt

yes you are allowed to use it

quite a lot of runners use AutoSizer, which is basically the same thing (achieves the same end result), so yeah, it's allowed.