Luca Troian, While I think Paul comes off a little aggressive, he is correct. DPI is meaningless when creating a computer generated image. What does matter is the output resolution and your field of view. The math is fairly simple as long as you know what you are setting up in advance.
I'm going to use small numbers for simplicity's sake, but the math is still the same for larger numbers. If you want to render a image of a box in 300 dpi and you want a image size to be 2x1 you must make the output resolution 600 pixels x 300 pixels and using your position in frame and the field of view slider you can create a image that fits your parameters. There might be some image size scaling in Photoshop that will be required but that's one step at most. A DPI selector will only be necessary from Enscapes point of view if they add a new output option that lets you base the image size on a in/cm width and height.
The only reason the blue box exists in Photoshop is because of the information in red box. Enscape doe not have image parameters based in real world dimensions(red box) so the blue box is not necessary.