Collectors should be aware that there was an EXACT REPRINT of BATMAN COMICS # 1,  published in 1974 (but Dated 1940) under DC Comics' FAMOUS FIRST EDITION line.  Originally, the comic had a stiff outer cover identifying it as a reprint.  When the outer cover is removed, it is IDENTICAL to the original book, except that it is OVERSIZED 10" long and 13 1/2 inches tall.  There have been many instances over the years where a non-comic collector has been fooled by one of these reprints.  When I owned a comic book store, I got at least one call per month from someone with an oversized reprint of either Action Comics # 1 (1st Superman), Detective Comics # 27 (1st Batman), Sensation Comics # 1 (1st Wonder Woman title), Whiz Comics # 2 (# 1) (not identical to the original, but not identified as a reprint either), Batman # 1, or Superman # 1.

A genuine Batman Comics # 1 will be the same size as a normal Golden Age comic book, which is slightly smaller than a normal sized magazine

On eBay, you can sometimes spot the Famous First Edition version by the cover design.  An original Batman # 1 has a red building in the background of the lower left corner, and several red buildings on the lower right corner.  A Famous First Edition has a red building on the right hand side only.  The lower left corner of the reprint is yellow.