*Nods* Yeah, we don't know for certain who is taller. But if I'm being honest, I think we would be able to infer that if the two adult-sized Snape and Harry stood next to one another that Harry would be the taller of the two. But since it is not stated explicitly, this is one topic where I will stick my fingers in my ears and waltz around saying "Lalalalala I can't heaar you". ;D
Snape is just such a larger than life character. Everything about him just screams 'bigger' to me (and I totally know where your mind went, missy!). Plus all those years of being bombarded with images of Rickman and Radcliffe just combine to make it feel wrong to me when Harry is taller than Snape. So much so for me that it always temporarily throws me out of a fic and permanently out of any art regardless of artist or the technical merit of the piece. For me so much of art is visceral and taller!Harry screams WRONG to me even more than seeing an artist draw two right hands, for example. But that's just me. *shrugs*