you've never seen it? you're in for a treat!

Hmm, I'm reaching here, but I can imagine a context, e.g., after an imperative. So: go do this thing for me wa-ma anta illa sayr al-barid. Meaning, go do it, and pronto, like a pony express rider.

...alternatively given as 2 or 4 parsangs. sayr al-barid is presumably quick? quicker than on foot at least.

think it's more to do with the pace of the journey/traveling. barid refers to both the mule/mount carrying the messenger (lane suggests barid this sense comes from Hebrew for mule) and also the distance btwn way stations...

also Harun al-Rashid, Muraqqish al-Asghar/al-Akbar etc

ianal, but is it possible that in this instance it's because it's part of his name? Zayd the Tall, a la Erik the Red, Pipin the Short, Alexander the Great? So "That's Zaid the Tall," tall being an attribute/description that is part of his name/the noun that he is.