The poster measures the regular size of 14" x 22" and was originally available to theatre owners for 7¢ apiece in the Snow White pressbook (p.10). However, this particular version with the reddish background is a rarity. In fact the folks at Heritage Auctions have never come across one before.
Much more common is the blue style window card which is dated 1938.
The red style is a first edition version of the poster and is copyrighted 1937.
In the lower right-hand corner is the Walt Disney Productions "©WDP" imprint. Below it is a small 5-digit number, 13141, believed to be an RKO print reference number.
This particular window card was used by the Kewaunee Theatre for their screenings on May 28-30, 1938. Where was the Kewaunee Theatre? A quick Google search reveals that a movie house by this name did exist at the time in the small town of Kewaunee, Wisconsin.
Images via Heritage Auctions with additional photos courtesy of the Brad M. Collection.
The poster is now available for preview on the Heritage website.
Also see a couple more examples of Snow White window cards in an earlier post.