I think it was 13X27"... Not so sure.
One can purchase a Beatles poster from the following sources: All Posters, Do The Green Thing, Amazon, Pop Art UK, eBay, Poster Gully, Your Beatles Memory, Beatles Number 9, Handy Posters.
You click on the poster then still holding the poster click on the yellow puffle
Yes, it's worth $300.00.
it depends if it is good or bad condition
original poster
Watch eBay listings. Major auction houses: Southerbys, Butterfields & Butterfields. Be careful, there are a LOT of forged Beatles items. The Beatles themselves used to sign for the other Beatles as well. An authentic signed poster (if one exists, as I've never seen one) would be worth tens of thousands. If someone is selling one for $500 or something - it's most likely fake.
overpowered Edit: Original Poster
To buy a Beatles poster, the best place to look would be shops which mainly sell posters, which can be either online shops such as AllPosters, or physical shops more likely to be in larger towns or cities. Nowadays many of these shops also have a presence on websites such as eBay and Amazon.
You can't modify the image like you would with the original. It is sometimes possible to make changes in Photoshop or overlay blocks to cover the original etc. but it is not particularly practical.
I've heard it was a circus poster that Lennon saw.
"Being for the Benefit of Mr Kite" from the Sgt Pepper album.
its under the poster with the instrument on the right wall