It's the exact same as windowed mode, but in full-screen.
Also the only difference you'll find is things that don't use the full resolution of your monitor will be effected, for example. If your desktop is 1920x1080 it will look the same unless you make it smaller.
Nvidia:
1. Right click your desktop and open the Nvidia control panel.
2. Under display click adjust desktop size and position.
3. Under the scaling tab make sure perform scaling has GPU selected.
4. Then under select a scaling mode check no scaling.
You're done!
AMD:
Well I don't have any AMD machines, however find your way to this menu and check the use centered timings.
^ If anyone can message me the steps I'll give them credit!
Then open Osu! tick full-screen and play with your desired resolution!