The default width for Moodle notification dialoguss is 400px, even on screens that are narrower than 400px. (Mobile phones have screens narrower than 400px, in portrait mode.)
You can fix this in code that is providing the dialogue, such as the following, which looks pretty good:
I think it might be preferable if this behaviour were default - i.e. if no width is specified then rather than defaulting to 400, it should use (400px or 50px less than width of window, whichever is smaller).
I'll stick a patch up for this. Not sure if it is a bug or improvement... The code is different for master/2.6 vs. 2.5 because stuff got moved around, so I'll just code it for master atm.