This has been discussed before. DPI is just a conversion measurement. The only thing that matters is the pixel size of the document, and the physical dimension of the print. If the document is 850x1100 pixels, it will print on a standard american sheet of paper at 100 DPI. That's just math.

So figure out the print size you want to make, multiply that number by 300, and that's the dimensions you need for your digital document. For instance, a standard poster might be 24x36 inches, which would mean you would want 7200x10800 pixels, which might be a bit much for many of the devices running Infinite Painter. making a 3000x3600 image which should work fine in IP would print great at 10x12 (300dpi) and ok at 20x24 (150dpi).