you don't have to be overly muscular per-say to have authority...
I'm fine with Reed having a less then traditional superhero body type
(yes, he's been drawn like that in the past, but, everyone in comics are over exaggerated) people should respect him for his mind, first and foremost
an given his power set (and more importantly his use of intelligence/planing/inventions/tech) he's not really required to be buff (fit/healthy yes, but, doesn't need to be a body builder) I mean as man of science he should have an understanding of the importance of a healthy regulated fitness routine/ and the knowledge of how to maintain a balance diet...
but, he's also the type who gets caught up in his work/loses track of time and sometimes forgets to take care of himself (so I can't picture him spending countless hours in the gym daily lol)
he should be more of an example of brains over brawns
maybe in the occasion of him needing to intimidate someone he use's his stretching to get bigger then them, but, for the most part he's more the type to try to talk/think his way out of most situations before resorting to brute force