It's really hard to extrapolate decent grammar from pop. But here goes... 1) It simply sounds better that way. 2) The fragment is adjectival, so it doesn't take on plurality. It's simply placed in an unusual position.

If he meant 30 feet high in the air, you are correct, but he means a 30 foot high poster. (like a 10 gallon jug, or a 30 inch TV). However, even then it probably should be written as "your 30 foot high poster." It's a type of positioning used often in poetry. Instead of saying "he is a loyal and honest man" people might place the definition of what they are describing after the intended noun: so you would say "He is a man, loyal and honest." Try to think of it as having commas between poster and 30 feet high. The only Other time I would think you would hear it, would be if someone was to rhyme off descriptions of something in real time. (i.e. I like this, cake, soft, moist, hmm, so good.) instead of "I like how soft and moist this cake is, it's very good." This is great, honest, straightforward, I really like the presentation, clean, well thought out. They are not 'real' sentences, they are sentence fragments, that's why they do not resemble grammatical sentences, because they are not. ALSO: It's poetry so it doesn't follow the same structure as prose. So I called up the Captain, [AND SAID]"Please bring me my wine" He said, "We haven't had that spirit here since nineteen sixty nine" And still those voices are calling from far away, [IN CORRECT PROSE IT WOULD BE "WERE CALLING"] [THEY WOULD WAKE]Wake you up in the middle of the night Just to hear them say... [AND YOU WOULD HEAR THEM SAY:]

As Peachey says, the poster is a high one. It is a - thirty foot high - poster.