So on a monitor that has multi resolution input capability the focus or dot size must be adjusted by the control circuitry, or the same line 'printed' more than once.
Going way back b&w tubes just had a coating of phosphor to produce the image, so definition just depended on the raster and video bandwidth, however colour tubes have a shadow mask as mentioned, this gives you a maximum number of pixels on the screen.
Image 'quality' isnt just about pixels, noise, colour decoding and other processes in a Tv also affect the image.
If you try it send us a pic or vid I'd be interested to see what would turn out like.