Hi there,
I have an small intuos 5 paired with a 27" monitor and I feel the pad is way too small to work accurately, specially with smooth traces both in Zbrush and Photoshop.
I found a guy selling an old Wacom Intuos 3 12x19 (HUGE!) for around $100. I’m on a budget so I can’t really expend much right now.
Do you think this tablet would be way too big? I’m afraid I’ll have to move my arm too much and end up with a tired shoulder quickly. What do you think?
This is a pretty individual question.
I thought the same as you do now.
A bigger tablet was a bit better, but much more to move your arm all day long.
Best if you can test before buying.
I have a small Intuos pro for travelling.
You have to zoom more. If one need precision, I increase lazy mouse.
This way I am feeling fine. Other folks don’t need that.
One mate is just so good at stroking. Makes me mad.
So I went all the way from small to big to cintiq back and forth.
For me personally I found peace with an used old Cintiq.
Others don’t like that at all. Lost of workspace if you have an old cintiq like me not to mention.
I am saying that not to tease you.
I guess if I would have concentrated on one specific tablet, I would have saved a lot of time.
This way I was searching for the wonder tool forgetting that the strongest and easiest to be trained part is sitting before the tablet.
Try to test it before you buy.
Just to find peace.
Good luck.
a 12x19 is very big, much bigger than you’ll need for decent mapping. you can however restrict the mapped tablet area to something manageable. one thing to look out for is that tablets in those sizes (12x12 and up) usually are thicker (about 2 - 2.5 times if memory serves) than the more regular sizes. there’s also a bit of a slope to the drawing surface.
it’s big enough to put a keyboard, mousepad and coffee mug on there. throw on some figurines, too. 
i am currently using an intuos 5 M with a screen like that and find it a tiny bit too small to be honest. still getting used to it.
what i found a really nice size to work with a 27 inch screen and older wacom models is the 09x12 (also called A4). you can adjust the tablet mapping to get a good aspect ratio fit and you will not develop a tennis arm from using it. i think they also made wide versions of the intuos 3 tablets that might fit the aspect ratio a little better. i’d go for one step up from size M in any case.
I agree with Knacki. Much depends on a personal factor. I have friends who are comfortable with the smaller versions while I 'd never be able to adapt. As you have already advised you should try them for yourself. If you do not have the possibility seeking an online store that allows you the return if not satisfied, so that they can try it without risking losing money.
I always preferred the smaller versions. For me, the ability to easily transport them is an important factor. This is because they are always on the go! If one day I will have a fixed place to work probably I will opt for something bigger