There's an answer here on What tablet are the pros using which might be beneficial for you to have a peek at.
I would like to stress though that the size you get is entirely up to you and how comfortable you are drawing on a smaller or larger size. In addition to the size of your tablet relative to your screen size. Though you will undoubtedly get used to whatever tablet size you get, there are some considerations that will make it a more comfortable process.
Personally I use the medium sized Intuos Pro and have found it a treat with my 22" display.
The topic is discussed nicely here the part relative to size;
Choosing the right size
Before looking at products and prices, you need to decide on a tablet size. You don't want to feel the sting of buyer's remorse if you buy too small, or too big.
Based on my own experience, I recommend a one-third rule. That is, don't buy a tablet that is smaller than approximately one-third of the size of your screen. (I'm referring to diagonal measurement, which is the common way of categorising screen size.)
So, if you have a screen which is around the 17/18/19 inch size, a 6 inch tablet is perfect. It might even be ok on a 20 or 21 inch screen. But if your screen is up there in the 22/23/24 inch category, I recommend going for an 8 inch tablet instead.
And if you have a whopping 27 or 30 inch screen, or if you have dual screens, then you should choose a tablet that's 10 inches, or even larger.
Why avoid a tablet that's too small? Well, it's about the precision. I've found that if my tablet's diagonal is smaller than one-third of my screen's diagonal, I start to lose the pixel-by-pixel precision for my editing work.
However, I hasten to point out that many people are using tablets that are smaller than one-third. One person told me that they used a 6 inch tablet on their 27 inch iMac, and were perfectly satisfied with it. Admittedly, my retouching and restoration work requires very precise pixel manipulation, so it's possible that my personal experience is not relevant to the broader photographic community. Or maybe I just have an unsteadier hand than most people!
If you're a member of a photographic forum or club, I encourage you to ask other people about what tablet-to-screen size ratio they use, to help your decision.
The opposite problem
Finally, I must mention that it is possible to buy too big. Based on everything I just said above, you might assume that if big is good, then even bigger must be even better! However, it isn't necessarily so. If your tablet is too big, your forearm swings around like a windscreen wiper trying to get from one side of the screen to the other, and it's uncomfortable.
Mind you, any decent tablet will allow you to reduce its effective area in the Control Panel, so you can prevent discomfort easily.
But in my opinion, you don't need your tablet to be any larger than half the size of your screen.