To render an image based on the size of the final image for 300 DPI, simply multiply the desired physical dimensions by 300. E.g., if you want an 8” x 10” print, then you want a 2400 pixel by 3000 pixel render.
Keep in mind, DAZ Studio can only render 10000 pixels by 10000 pixels.
Don’t forget to set the resize options so that the pixel size remains fixed. It’s way too easy — I know, I’ve done it — to leave the resize free to wreak havoc and turn your 300dpi A4 print into a 300dpi postage stamp.
Another option, use a dedicated image viewer utility. I use Irfanview (in Windows, don’t know if there’s a Mac version), and it allows you to load pics and set the DPI value directly without going anywhere near the resize function. Smaller and faster than Photoshop, too.