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 Action Comics # 1 will be the same size as a normal Golden Age comic book, which is slightly smaller than a normal sized magazine.
There are also several modern reprints of Action Comics # 1, which are sometimes "aged" to look like they are older than they are. There were reprints published in 1976, 1983, 1987 (Nestle Quick 50 cent cover), and 1993. These can be distinguished by the fact that they reprint the Superman story ONLY. Some of the reprints have paper covers instead of glossy covers. The original comic has 64 interior pages plus the front and back covers.
The exception to this is the 1998 US Postal Service reprint of Action Comics # 1, which reprints the entire book.