Open your scan in Photoshop, and note the size. Note the actual dimensions and the resolution, or the absolute pixel size (it's less math). Say your scan is 1600 x 1200 pixels (5.3" x 4" @ 300ppi). Render your Bryce image to be at least 1600 x 1200, and you are good to go. FYI: ppi doesn't matter at all until you actually try to print your image. At that point, you will need at least 150 ppi for printing on an inkjet, and 300 ppi for professional printing.