Is this graphic of molecules correct? First off, huge fan of chemistry, not a chemist – do be nice :D

Would an image like this make sense as an infographic for the molecular composition of the air? I especially wonder if the different sizes of the molecules (or do they all have the same size?) and the constellation make sense in theory.

Many thanks
No.  Most molecules of air have two atoms. 

So if you want to be quantitatively accurate, if you have 100 molecules in a graphic, it would show

78 dinitrogen (N-N) molecules 

21 dioxygen (O-O) molecules

1 argon atom (Ar) 

http://eesc.columbia.edu/courses/ees/slides/climate/table_1.html

And you could add some water (H-O-H) molecules if you want to depict humidity.