If a graphics card has different sizes of memory, the lowest value will be used, with the higher values being disregarded. Currently, scaling on consumer grade cards can be done using up to four cards.
Full towers can usually fit larger motherboards in sizes like ATX and micro ATX. The larger the case, the larger the motherboard, the larger the graphics card or multiple other components that will acquire case real-estate.
A graphics driver usually supports one or multiple cards by the same vendor, and has to be specifically written for an operating system. Additionally, the operating system or an extra software package may provide certain programming APIs for applications to perform 3D rendering.