I'm still stumped with regards how much you can actually draw on each size of active surface
usually with a wacom the drawing area of the tablet is mapped to the screen, so wherever you put the pen on the tablet, the cursor 'jumps' to the corresponding place on the screen; unlike a mouse where the cursor stays where you left it [but you can use a wacom in 'mouse mode']

the size of the tablet should be determined by the size at which you feel comfortable drawing without being too loose or cramped, assuming you use it in pen mode; in mouse mode it don't really matter most of the time

Surely things begin with a sketch, which to me is a series of lines, which to me is best suited to a vector app
why do you say this? [best suited to vector?]