John James Audubon created a series of pictures of birds, known as The Birds of America, originally published in the mid 19th century. There were 435 different images, each showing a bird or birds fully life-sized, which is why you see some birds, like the flamingo, in what seem to be contorted poses.  He was trying to fit a tremendous bird onto a page approximately 29" X 40".   The paper size is referred to as "Double Elephant" and  that name is often applied to various Audubon prints. It refers to the large format and does not necessarily imply anything about the age or quality of the print..

The original hand-colored engravings are very valuable and beautiful as well.  They are in the collections of many famous museums and libraries.  Unlike modern limited-edition prints, the original  Audubon prints were not "signed and numbered". They do include two numbers on each print, but these are part of a catalogue system, with no connection to edition size. 

Most of the original images were engraved by the workshop of Robert Havell and hand colored under his supervision.  Almost all reproductions of Audubon's bird pictures include Havell's name at the bottom, along with Audubon's.  That includes posters, calendar tops, pages from books, and  "art prints" of varying quality and value. JUST BECAUSE IT SAYS HAVELL, IT IS NOT NECESSARILY 150 YEARS OLD. Any copyright on these images has long expired, so there is nothing unscrupulous about reproducing them.

Audubon's birds have been reproduced in facsimile (full sized) editions several times, either partially, such as Alecto, Princeton, or Leipzig editions, or in full, such as the Amsterdam edition or the Abbeville Press. M. Bernard Loates has also produced a series of limited edition  prints based on Audubon's works, but they are sometimes altered slightly and they do not include the  attributing language usually seen at the bottom, with no  reference to Audubon or  Havell.  Still, they are of very good quality and can often be bought at bargain prices on eBay.

The distinguishing factor of the original work and its best reproductions is that the birds are life-sized.  When you look at a picture that someone thinks may be an "original" Audubon print, the first and easiest test is to see if the bird is life-sized.  Because many of the prints were produced with wide margins that may have been trimmed, "sheet size" may or may not help.  Watermarks in the paper help to identify several editions, but these may not  be visible if the print is framed.

Once you have bought your print, you may be tempted to trim it.  While this is often considered unadvisable or damaging to the value of the print, consider these things:
1) Will it be more attractive with  smaller margins?
2) Are you likely to get more enjoyment out of it with smaller margins?
3) Can you afford a nicer framing job if you trim it slightly?
 4)  How much value are you talking about anyway?
 Framing a $50 Amsterdam print will likely cost you over $100, regardless of its eventual size.  Unless you  expect to sell it, chances are you will be better off trimming it a little, hanging it, and enjoying it. Let your grandchildren worry about the diminished value.

There are several sellers on eBay who regularly offer Audubon prints. Their descriptions are detailed and quite informative. Check their feedback and you will see that they are honest, decent people.  Of course, there are others who collect mostly, but may sell on occasion. Chances are, they have bought from the same dealers. And then there are other sellers who deal mostly in posters and their prices reflect the difference. Some sellers may  encourage you to misunderstand what they actually have for sale. Be sure to ask questions and  to assure yourself that the  value YOU place on the item will be greater than the price you must pay.