Besides the listed site for NF campgrounds, try finding MOON'S travel guides. Here is a link to one: Moon Pacific Northwest Camping - Moon Travel Guides
They have a lot of info, and pad sizes within. Don't always cover all possible campsites, or always give lengths, but very inclusive. There quite a few books, and they can be repetitive, but the one link posted is a great one for WA & OR. There is also one that covers ID, MT, WY and the Nat Parks done just like all the others with great info.
I will say that some of the lengths listed for spots we have been to, weren't entirely accurate, and did somewhat ere on the side of short!!! But have found most to be fairly on track.
We found these at our local library, which is a good way to see if you want to buy one for "permanent" use!!!