It seems to me that weight and size would be important in choosing spacefarers for many reasons; I came across this Wikipedia article where the Chinese astronauts (taikonauts) have to be between 50 and 70 kg, which makes sense to me, but other countries don't seem to stipulate a size requirement.
I would think that it would be easier and cheaper to get the person into space if they weigh less, and could survive on less food for a given time frame so you also save on the provisions.
Cramped conditions would be less of a problem.
All else being equal, is there any reason why sending bigger people into space would be an advantage?