My mental conception of Halflings has been informed by a few key pieces of art - most recently this one:
While they're not the stout Hobbits of old, they're also not the tiny dreadlocked humans Dungeons & Dragons has occasionally embraced. I like this depiction of them - their child-like proportions allows one to tell they're small, despite the lack of reference, yet their adult features allow one to distinguish them from children. I'll probably rely on illustrations like these to depict halflings in my campaigns, rather than the official art we've seen so far.