AE actually ignores dpi. It shows 72 dpi just to make print people feel more comfortable having a dpi value. What matters is the pixel dimensions of the output. For example, at 152 dpi, an image 1″ x 1″ would output as 152×152 pixels. Calcualate the size of the output image in inches and then multiply by 152 to get the output image size.
Jim Kanter,
Digital Film Institute
http://www.dfilminst.com