Thomas Edison decided when the first movie's where being shown that the posters should be 27x41. Posters where printed in that size untill the late 80's early 90's then it was changed to 27x40. Not all original posters will be exactly 27x41 or 27x40 they can be a little off . If your poster does not quite measure up don't toss it thinking it's a reprint. Now saying that, size is a good indicator if a poster is real. Most posters came folded to the theaters untill the late 80's. Some rolled  posters did come out and collectors estimate about 5% of the posters made came rolled. So if a seller is constantly every week selling that rare title rolled you can assume it is a reprint.Many ebay sellers list reprints as original. Do not think that if a movie poster is in the original category it's real. Look at a sellers feedback do the people who buy from that seller have low feedback this explain's why people who sell fakes get away with it. New buyers are not informed and leave a great feedback. Later when they get into collecting they notice they got a fake, and it is too late to leave the feedback that the seller should of got. Now there are a few sellers that do come up with rare titles a lot and are legit sellers. The reason they have these posters is that they are constantly working at finding these posters. They are in the minority. How can i tell my my poster is a fake well if it was printed before 1985 and is 27x40 99.99 percent fake , No nss logo on a nss issued poster, Not all nss posters have a nss number most do, fuzzy poster, no nss stamp on the back they did this alot in the 60's, No gau logo, Is the price too good to be true a Nightmare before christmas for 9.99 buy it now. Where is the seller located just keep looking there is a pattern of where the fakes come from. Now what are  Re-issues- Re Releases and Bootleg's . Bootleg's are not issued by the studio and have no value there are exceptions but in general they are useless to collectors. Re-Release posters are different they can be collectible Gone with the Wind is a perfect example a good way to tell a Re-Release poster is the R in the nss number. A Re-release poster sometimes has better artwork then the first poster. Decoded nss number r823045 it means re-released 1982 and the 45th movie that year. Older reissues by the studio's can be collectible they should not be confused with a bootleg.There are some bootleg's posters out there that fool even the best expert's and they are still fake so do home work look up the poster. Ask sellers with great reps. Is there a zip code on that really old poster. Zip codes came out in 1963 that is a good tell on older reprints. Paper is also a good way to tell does it look new minty white on a insert, thick when it should be thin or vise versa. Now in the 80's there are original posters that have thick stock and can be glossy no one knows why but they are out there. Is there a miramax stamp on the back IT IS A FAKE. Now sellers like to talk about re-issues by the studio.  A re-issue can be very collectible a re-issue is not a bootleg.  Watch out there are sellers that will not tell you it is a re-issue and if you are trying to buy an original you will be disappointed. If you are not sure about a poster don't buy it . You will find it again it may take time it will come. Impulse control is paramount if you want to save money.Also one thing i have noticed of late some sellers are selling original poster with a stock pic of a perfect poster. I would be wary of buying them you do not know what the poster looks like and by the time you buy it's too late. Here is a list of posters that people sell a ton of reprints this is not complete just a start.

Here are some terms to help you understand what i mean

Bootleg- is a non studio poster printed to fool a collector

Original poster- issued by the movie studio to promote the film also theater chains like lowe's did printing with permission of the studio and about 30 other theaters had similar agreements.

Re Release the second of third time the movie was in the theater you will find a R in the nss number

Re issue is a poster that the studio gave permission to print and some older ones are desirable to collectors

BACK TO THE FUTURE , A NIGHTMARE BEFORE CHRISTMAS, PULP FICTION , ALL THREE STAR WARS MOVIES ,THE  USUAL SUSPECTS, A CLOCKWORK ORANGE , SCARFACE, THE CROW, DONNIE DARKO, MATRIX, SHAWSHANK REDEMPTION , SPIDERMAN ADVANCE, ALL THREE INDIANA JONES POSTERS, GOODFELLAS, THE GODFATHER, ARMY OF DARKNESS,
FRIDAY THE 13TH, AMERICAN GRAFFITI , KILL BILL, BEETLEJUICE, This is just a sample of titles not a complete list by no means

USEFUL LINKS

CINEMASTERPIECES INFO ON FAKES

Here is a link for the group on bootleg posters there are some very well known poster sellers who are members  http://groups.ebay.com/forum.jspa?forumID=100043974

 A reservoir dogs reprint to the left note the white lettering on the top the space is to small

   The pulp fiction international real one is to the left note the colors

    The october advance the reprint is the left poster the reds are quite different

    This is just to easy the fake stands out the real one to the left is nss issued with number

         The poster to the left is a fake note the color and credits the right one has been linen backed

Just a few examples of how a fake can seem to a new collector to be quite real and they get scammed every day the pics for the originals came from a few members emovieposter.com , kymar, cinemasterpieces

I would like to thank everyone who has voted this guide thanks

I would also like to thank Movie Poster Authentication ebay id fromdusktilldawn for his web site that gave me the idea for this guide.