Frenchy,
I will heartily disagree with you. I am a professional and I have used both the Graphire and the Intous2. I have used the 4x5 and the A4 sized tablets. There is a noticable difference in the quality of your line due to differences in tablets. One such difference is the amount of pressure the tablet registers (512 vs. 1024, in most cases). There are several other factors as well:
1 - Does the tablet pen need a battery?
2 - Does the tablet pen have a cord?
3 - How many buttons does the pen have?
4 - Does the tablet pen use tilting?
… and more.
You don’t have to buy a Wacom. I was producing book cover illustrations without one. But, at this current time, there really is not substitute when it comes to quality … not just of the tablet itself, of the line you draw with the tablet. The Wacom is smooth and fast. As a result, so will your line be (in comparison to other tablets).
As far as size, it certainly does matter. It’s not just your brain. It is your personal style. Some people draw on large canvases with large, sweeping motions. Some (like me) tend to draw small. It all depends on how you draw details. Just like working in ZBrush. Imagine adding details if you could not zoom in on an area? Imagine drawing details if all you have is a little space in which to draw?
As stated, I tend to draw small. So the A4 sized Wacom is more than enough for me. The large Wacoms (and I had one at a place used to work a few years back) were way too big for me. But that’s me. When buying a tablet you need to see what is right for you (size-wise) and then purchase accordingly.