I agree. As a native english speaker, I would never say "a tall one", and I would have no idea what someone meant if they said "a tall one". I think "a tall person" and/or "something tall", depending on how Hungarian speakers use it.
Why not valamit magas? Wouldn't the adjective go back to being an adjective and then would valami not need to be a noun in the accusative? I feel like I'm missing something here...