In your osu folder, there should be a .cfg file named something like
"Osu!.[Something related to your computer name].cfg"
Open this file (With notepad when asked what app to use/etc), CTRL+F to search for the following lines:
"WidthFullscreen =
HeightFullscreen ="
Change the values to the desired resolution.
1920x1080 if you have a 16:9 monitor, if that's too big try 1280x720 and go down from there.
You can also CTRL+F to search "Fullscreen =", find "Fullscreen = 1" (if fullscreen is enabled), change the value to 0 to disable it, save, close notepad, open osu! and go from there.
(this should work, i don't see why not but i donno xD)