I managed to get the height of the window extended by loading a song while osu is minimized in the taskbar. osu.cfg settings were normal (ie. 640x480).
I tried to do it again but it didn't work.
edit: I forgot to take a screenshot, but I think with my case, it was the game itself that shrunk, not the window enlarging, because the beatmap I was playing back then had weird croppings for sliders, which don't exist now.
edit2: I figured out how to get this. Put osu into a song in edit mode, minimize osu, and load an osz. It'll jump to play mode without readjusting height.