I am creating a movie poster and am wondering what the correct project size is for this. What are the dimansions and dpi that I should use to create a movie poster than will be scale of real movie posters?
DPI depends on your printer. DPI of about 220-250 is appropriate for inkjet printers, for example. If you're having the poster printed professionally, contact your print service and ask them what DPI they recommend for their equipment.
(Another factor, somewhat less important, is that larger prints usually require less DPI because being seen from farther away, they don't need so much fine detail. Billboards, for example, use a DPI around 50.)
As for size, there is no single movie poster size. The standard size that's used for posters in theatre lobbies and for first-run retail posters is something like 40in x 28in. Larger sizes are often used for theatre exteriors and promotional displays, and smaller sizes for retail, promotions, and other types of advertising. There's no fixed size or ratio, though, it's just up to the studio and their advertising department.