The actual size of the image generated depends on whether you are running in DPI mode (default), or you explicitly set a size, using PDFDraw.SetImageSize.
SetImageSize is the preferred way, as if you set the DPI to a high value, and happen to rasterize a PDF page that has large dimensions, say an engineering/architecture drawing, you can end up with very large memory consumption and a very large image output.
If you do use DPI, then PDF are in 1/72 scale. A default PDF page is 612, 792, which is 8.5"x11". So PDFDraw.SetDPI(300) would result in a image that is 2550x3300 pixels, or ~8MP.