My new plug-in Dpi2Pix converts dimensions in mm/cm or inches, to dimensions in pixels, using the print resolution in dpi. The size is defined directly in the render settings. You can choose predefined formats (A3, A4..., 2x3 inches, 4x6 inches...)
The plug-in works with C4D 6, 7 and 8 versions, with Mac and PC systems, in french and english languages.
More info and how to buy it http://3dweave.com/plugins/index.php
the price is very low (4 euros HT, 4.52$ without taxes)