I always pictured him with longish hair, actually. Like, the kind that is always covering his face that he could hide behind when he felt the need to. Just long enough for him to tie back on occasion, even. And I always pictured it as lighter than David Thewlis’ hair, too, more of a dirty blonde to light, sandy brown kind of colour, and a little bit thicker in his younger days, before the toll of adulthood poverty set in.
I always figured that his time as a student is when Remus would’ve been his most healthy, so I don’t see him as all that painfully thin, either. Certainly he’d be looking wasted around the full moon, but for the most part I think he had a build that could be described as at least pushing healthy.
As for clothes, as much as I hate to perpetuate the stereotype of teenage Remus looking like a teacher, I do see him wearing a lot of knitted sweaters and corduroy trousers… But I mainly see it as a matter of practicality, not style - op-shop specials, hand-me-downs, or hand-knitted things from his Mum. And in the warmer weather he’d be wearing faded t-shirts and the like.
Facial features wise, this is weird, but I don’t really have that strong a picture of teenage Remus in my head. The only thing that sticks out for me is that I feel like he had a wide mouth. Odd, I know, but there you have it!