Collectors should be aware that there was an EXACT REPRINT of Detective Comics # 27,  published in 1974 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 Detective Comics # 27 will be the same size as a normal Golden Age comic book, which is slightly smaller than a normal sized magazine.

There is also a 1984 reprint of Detective Comics # 27, produced as an Oreo Cookies giveaway.  This comic is 32 pages long instead of 68, and has 3 batman stories instead of 1.  (The others are from Detective # 38 - 1st Robin and Batman # 1 - 1st Joker).  the cover is paper instead of glossy, it says "collector's edition" on the cover, plus the cover price is "free" instead of 10 cents.  This is less frequently passed off as the genuine article.  It is harder to find and worth more money intact.