Wacom Intuos 5

I am also thinking about getting the Wacom Intuos 5 but cannot decide on a small or medium? I plan to use it for retouching portraits in Lightroom and PS on both a 15" Macbook and a 27" iMac...

Do any of you using medium sized tabs find you have to wave your arms around to get from one side of the screen to the other?

I have tried to find a shop I can try it in but there aren't any in my area!

Any advice appreciated!