it really depends on which mode and how you would use the tablet...
there are two modes to using the tablet: the screen mapped
pen mode, and the
mouse mode
in the
pen mode, the tablet is mapped to the visible screen, i.e. if you put the stylus to the top right of your tablet, the mouse pointer on your screen would move to the corresponding point, top right, on your screen...in this case, for a small tablet like the 4x5, a swipe of your wrist can move the mouse pointer across the whole screen...thus to use the 4x5 tablet in this mode would call for alot of control, and you would still not have as much precision as if you had a large tablet of the same series running in similar mode. that's what some have mentioned as being too magnified...
in the
mouse mode, the tablet is not mapped to the screen and the stylus acts like it would if it were a very skinny mouse ( ;p )... i.e. to move the mouse pointer to the left of the screen, you would have to push it towards the left, in a few strokes, like you would if you were doing same with a mouse...try moving the pointer from the left of the screen to the right with your mouse
...thus for this mode, the actual size of the tablet is not so important in the case of on-screen precision...
now, I am not advocating one mode over the other, it's a matter of personal preference, but as you can see, how you use it would affect the size of the tablet for you...and of course, a small Intuos 3 would still have the same precision as a Graphire or Intuos 2 that's 4 times it's size even in pen mode...if you have the control to make such fine movements that is...