If you are working in tablet mode (i.e you have a tablet connected and have checked the option in the config screen to allow tablet pressure to set the size of the brush) then you cannot paint with the mouse because the tablet pressure reading is overriding the size setting.
At least, that's how it is supposed to work. Not having a tablet means it's hard to test.
At least, that's how it is supposed to work. Not having a tablet means it's hard to test.