Does size matter?

Who was the largest world champion? Who is the biggest current grandmaster? Smallest?

Largest & smallest in terms of what? weight? Should hardly matter in chess - after all chess is about brains and not size.

Perhaps, the place to seek answers to this is at chesshistory.com/winter/index.html

what is interesting is that most of the world champions that i see in pics are quite thin and i find it interesting that would be the case.

It's hard to imagine a fat grandmaster.

Well, for one, Garry Kasparov was an avid soccer player....I'd say that most elite players are in very good shape.

People generally perform their best when in good health.  But of course, there is an answer for anything on google :)

Studies have linked physical activity to mental ability; though this hardly disqualifies a person from being a top notch chess player if they're not in shape.

Just my (totally unresearched) opinion.

To be in your best mental condition... you must be properly nourished.

To be in your best physical condition... you must be properly nourished. 

Therefore - Top chess players are likely well nourished... and so... are likely in good shape :).

Just because you are well nourished does not mean you are physically fit even though good nutrition is a contributing factor to good physical health.