Here it comessss
Disable windows desktop composition and windows themes (This fixes windows latency, this is only if you insist on playing windowed);
UNCHECK windows ink, this fixes the problems with tablet lag as buny is saying (no other external methods needed);
Try windowed 1440x900 @480fps; Map your SCREEN AREA to be within the osu! window;
While having force proportions on adjust your TABLET AREA HEIGHT to what you find to be comfortable. (The width should adjust accordingly).
Play more and don't change it unless it's causing you problems.
That will give you a wider range to test with until you find something comfortable. People just set it to the 1920x1080 screen area and assume they're playing full screen/full tablet when it really doesn't work that way; they're actually using a pretty small area.
I play full screen (24" monitor, pushed back so it's 'smaller') full tablet (8"x5") and can say that the most important thing is to maintain aspect ratios; size is just preference.