If it's purely length you're after, I seem to remember Heathcote Williams published a long poem about Whales in the 80's called "Whale Nation". In Googling to try to find out more, I turned up another published, (I think) afterwards, called "Sacred Elephant".
They're both substantial - "Whale Nation" was, I think published, on it's own, in paperback and ran to at least 75 pages, although there were illustrations and photographs, as well.
I'm afraid I can't vouch for the content of either. I read a few extracts from "Whale Nation" around the time it was published but I can't remember very much about it.
Hope this helps...