I think that the Hosts maybe didn't realize they had that requirement, until they saw the nominees for Caranthir, and had the 'these guys are all too skinny/wimpy'! reaction.
The goal is to make Caranthir look like a bruiser. Not necessarily super tall (Maedhros, Turgon, and Finrod are all 6'5", so he can be shorter than them!), but like he has the frame to back up his brash fighting words. So, maybe a guy who is at least 6' and has broad shoulders or some bulk to his arms. He doesn't have to be extra built for that; we're not looking for an American football lineman here. Just a rugby player.
So, going through the roster of the Marvel films...well, this is a combination of casting choices, costuming choices, and acting. After all, while Bruce Banner doesn't come across as a guy who would be any good in a fight, Mark Ruffalo likely *could* play a much tougher guy. And Loki is only small in comparison to his brother. So, 'imposing' is at least partially about costume and acting decisions, not simply a matter of casting. But you have to start with the casting, I guess.
Not Thor, not Drax, not Thanos - these guys are too big.
Not Loki, not Bruce Banner, not Tony Stark, not Vision - these guys don't come across as physically imposing.
'Just right' - maybe something more along the lines of Captain America, the Winter Soldier, Black Panther, or maybe even Star Lord
I realize that 'lithe' is a typical way of thinking of elves, but maybe shift a bit more towards action hero/villain physique.