Hi there!
The size really just depends on what type of monitor and what size you are going to work on, and how you generally are more accustomed to drawing on paper. For example, if you are used to drawing large, a larger tablet will make you feel less like you are scribbling mini-dots, but if your are more like me, and start a drawing or anything of that sort, and you have no idea what you are about to do, and just sketch really small things, the whole “less is more” concept becomes SOOOO true.
Also, the type of tablet you buy is strictly related to your funds… if you need pressure sensitivity, tilt sensitivity and useful stuff like that, for use in photoshop, or painter, I really recommend you invest a bit more in either a graphire or an intuos (rather the later).
I personally have a wacom intuos 3 A6 wide, limited edition tablet, in shiny piano black. It is about the size of a 16 inch laptop, so it only makes sense that it’s almost one on one proportionally speaking with my screen especially since it’s widescreen. As I mentioned before I am totally unused to doing a clean drawing from start to end, so the size and slow evolving drawings were a major setback to me, but I really think that for “really detailed illustrations” as you describe them, a similar size is just perfect and with A LOT of practice you will achieve just DEVASTATINGLY astonishing images and illustrations. I’m still at the “A LOT of practice” stage, but BEWARE , I WILL FINISH MY PORTFOLIO IN FEBRUARY and you might just find one of 50 images that is pretty decent. 
Hope this helped, and good luck with your new tablet!
p.s: If you have any further questions feel free to ask.
Best regards,
Contrastblack.
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