These are the most common posters you'll come across, and are usually extremely desirable since they will very likely be country of origin depending on the film. In other words they were printed where the film was made. In the same way, Italian posters can be very desirable for Italian films such as A Fistful of Dollars or The Good, the Bad and the Ugly. Below is a summary of the most common formats.
One-sheet 27x40 (27"x41" for most older posters and some newer ones) - the most popular format you'll find. Always printed on paper stock, the quality of which has improved greatly over the years. Up until the late 1970s these were mainly issued machine-folded to cinemas (most commonly with 1 vertical fold, 3 horizontal). After this time some began to be issued rolled, and now all are sent in this way.
30x40 printed on card stock and usually featuring the same artwork as the one-sheet. Always issued rolled, hence if in good condition are very desirable for older films.
40"x60" - printed on card stock and usually featuring the same artwork as the one-sheet. For some the large size is a great showpiece, but the dimensions make storage and framing a problem. Always issued rolled, hence if in good condition are very desirable for older films.
Half-sheet 22 x 28 - printed on a card stock. Sometimes featured different artwork to the One-sheet, and is in a compact landscape form which makes it ideal for framing. Some were issued rolled, some folded, but with card stock rolled is much more desirable.
Insert 14x36 printed on card stock, sometimes featuring cropped artwork from the one-sheet design. In my opinion a great size, easy to frame and great to display. Sometimes issued rolled, otherwise folded (usually with 2 horizontal folds).
Window Card 14x22 printed on card stock, usually featuring the same artwork as the one-sheet. These have a large blank border at the top which was to be used to write in the showing times of the film at the local cinema. For many people having this blank (or having it with neat writing!) is preferable. Usually issued flat to the cinemas, so avoid folded cards.
3-sheet 41x81 printed on paper stock. Usually carry a unique design, and look spectacular on any wall (if your wall is big enough). However they are very costly to frame owing to their large size. Issued folded, sometimes in 2 separate pieces.
6-sheet 81x81- printed on paper stock. Usually carry a unique design, but extremely large so virtually impossible to frame. Issued folded, usually in 2 or 4 pieces.
Lobby Card 11x14 printed on card stock, usually issued in a set of 8 to the cinema and typically featured a title card and 7 scene cards. It can be very difficult to find a complete set for most older films. Perfect size for easy framing. Always flat, never folded.
Mini Lobby Card 8x10 as above but smaller and sometimes with an entirely different layout.