That's a reference to poster size. A typical movie poster (that you'd see at a theater) is called a one sheet (approx. 27 inches by 40 inches today). One Sheets are usually displayed vertically, as you increase in size the actually poster may be displayed horizontally.
A three sheet poster is about 3 times the size of a one sheet, about 41 inches by 81 inches. An 8 sheet would be roughly 8 times the size of a one sheet, at that size it's probably a poster intended to be used on a billboard. Bigger posters used to be a common occurrence (and of course billboard sized posters are still used today), but I think the studios typically stick with one sheets nowadays and use standees for the big stuff in theater lobbies.
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