All(?) pop up window dialogs in Muse CC 2014 seem to be approximately three times the content width and twice the content height (with the content in top left corner).
This means the new site box takes up most of my screen, the add tablet layout, add phone layout pop-up windows look silly and the edit > preferences window is unusable as it's larger than my screen .
Running on Windows 8.1 (with dual screens although disabling one made no apparent difference!)