I suppose with touchscreen you
can do some basic editing. But going beyond the quality of finger painting, or doing anything smaller, would likely take more effort with touch than without. Possibly depending on how well pinch-to-zoom works. Not being able to use a fine pen is actually the
one thing I'm really, really missing with my iPad.
I've been working with Wacom tablets for 15 years and I can't do graphics without one at all anymore (I believe the newer models have pen
and finger use, but I haven't tried that). Once you get used to it, it becomes second nature and you know where to put the pen exactly without even looking. They are pricey but offer an equal quality and I can wholeheartedly recommend them. I've tried some cheaper knockoffs but you get what you pay for and those weren't worth it, as their grid resolution was so coarse that the cursor was often twitching between pixels.
I have an Intuos Din A3 Wide at work and another at home (which I got during a great Black Friday Amazon deal) and that's the perfect size for me. Din A4 could do for portable work, I guess, and the mobile Cintiqs are probably fine as well, albeit sadly have the "wrong" OS' for me.
I also have a 24" Cintiq and love it (except for the price! T'was a rather mad expenditure). But going beyond 24" for touch would put quite a strain on your arm. Mine has traveled miles and miles on A3 already (until the Wacom driver stopped counting distances
). Also, the Cintiq is bigger than a regular 24" screen and takes two people to lift onto the table, so quite a beast.
One drawback you get with touch or pen screens is, that you cover part of your image with your hand/pen. Nothing too bad, but screenless tablets are without that limitation.
/edit: note how the Lenovo 27" advertises its "Multi-user mode"
/edit2: 27" screens
are a great size to work with, I just have doubts about running my arm along one.