Jordan mentioned "A Cloud in Trousers." Here are poems "of a certain length" that I like:

Ill Seen Ill Said [not a poem maybe? but who cares?]
Biotherm
The Morning of the Poem [and all the rest of JS' longer poems]
Paradise
Don de la ebriedad
A Handbook of Surfing
Piedra del sol
Radi os
Intravenus
Paisaje con pájaros amarillos
Many poems by Kenneth Koch in this category, too numerous to name
All of Basho's renga sequences

I like the "long poem" that's actually fairly short. Something you can read in an hour. In other words, not the Cantos or the Canterbury Tales or Paradise Lost or the Iliad. But not a poem that fits on one or two pages and can be read in five minutes. They are not long poems but poems "of a certain length." 20 or 30 pages is ideal. Read these and you will be happy. Very very happy. Just thinking that these poems exist makes me happy. If you read these and are still not happy I will buy you a beer at the AWP.