I'm not necessarily opposed to large trim sizes, but I've never seen anything approach 8.5 X 11 that wasn't hardcover. Usually, a big that big is a reference work of some kind. I've seen a few "collected works volumes" in hardcover with large trim sizes, but pretty much all the other really huge ones are reference books.
It's true that a large trim size would reduce the number of pages, but it's so different from what people would expect that I'd be wary of it.
Do you usually make a lot of sales in paperback? If not, I'm not sure I see the point of a huge paperback series volume. Only if you were a big seller in paperbacks would it make sense, and even then I'd split it into smaller volumes as others have suggested.