...I don't know my mouse speed, but desktop speed is right in the middle (since I use my laptop's trackpoint), and osu! multiplier is 1.0... As with everyone else, I use a program for settings on my mouse instead of Windows but...
I use mouse acceleration. And it rocks my socks.
I wouldn't see why I would turn it off for osu! or have a different setting, if I'm used to clicking stuff quickly outside of game, then I'll always be used to mouse speed ingame.
I don't know the exact dpi, but it takes about 4 cm (1.6 inches) at no speed, and about 0.7 cm (0.28 inches) at max speed to cross this 12inch screen.