DPI is dots per inch and refers to printers. Doesn't mean a lot in Reality.

You need to know what PPI (pixels per inch) you want, this changes with the optimum distance the print is viewed from.

A billboard poster could have a different PPI than a magazine advert but may have the same number of pixels.

To change the PPI, you just have to do the math.

Image size = physical size (inches) * desired PPI

So if you want a 6"x 4" photo at 200PPI, you want to render at 1200 x 800 pixels. For a 10x8 it would be 2000 x 1600 pixels.

I would suggest that you do some research on the settings people have used to print images similar to your desired result and go from there.