"Click on the picture to view it in full" would be ok if you were only seeing a portion of the picture (a single person or the upper left corner) and after clicking you see the whole picture "Click on the picture to view it in full size" sounds really odd to me. Try: "Click on the picture for a full size version"

"Click the picture to view full size." or "Click (the) thumbnail to view full size" as you'd usually have a lot of small thumbnail images on a page and click each one, to view a larger image. Either that or have the images open in a lightbox. http://lokeshdhakar.com/projects/lightbox2/

I suggest you use “in full size” (if that’s what you mean). I have never heard “to view it in full” used to refer to viewing something in it’s full size. “in full” means “completely”, which is not necessarily the same as “in full size” (part of the picture could be hidden for instance). Also, since Eliah mentions it, “full screen” is not the same as “full size”. It’s entirely possible to view a very small picture in full screen.

Full screen is most commonly used. Both examples would be understood though

I prefer "in full size" but "in full" is a valid abbreviation.