The reason this is hard is because of the conceptual difference between how big something is and how big it appears . I'll break this down as deeply as I can. The salient points: The Roll20 grid is measured in Units. 1 Grid Square = 1 Unit If you want your map to easily align to the grid, be certain your image's dimensions are in whole Units. Units can be whatever size you like. Roll20 suggests 70 pixels per unit for maps, or 140 pixels per unit for tokens. Larger sizes have more detail per Grid Square but lead to bigger files. For each of length and height, determine the size as: (Number of Units x Unit size (70 for maps, 140 for tokens, generally) Example: A 2x2 ogre token would be 2 Units x 140 pixels = 280 pixels wide by 280 pixels tall. Example: A creek bed north to south that is 8 Units across by 20 Units tall would be 560 pixels (8 Units x 70 pixels) by 1400 pixels (20 Units x 70 pixels) Discussion: Roll20 creates a grid that is so many Units wide by tall. On screen at 100% zoom, these Units are 70 pixels by 70 pixels. At 200% zoom, they are 140 pixels by 140 pixels. In your game, the Units of your map may be assigned some other measurement scale like feet, and so the interface allows you to measure 3 Units as 15 feet, but the size of that span on screen will depend on the zoom. At 100% zoom, 15 feet == 3 Units == 210 pixels. At 200% zoom, 15 feet == 3 Units == 420 pixels, at 50% zoom, 15 feet == 3 Units == 105 pixels. Similarly, Photoshop creates images. Images have a size in pixels. Just like Roll20, Photoshop has other measurements it can use. Because Photoshop was originally designed for use on photos, where the size of a physical print is what the primary user is concerned about, the default measurement it uses is Pixels per Inch. Pixels per Inch on the screen are intended to translate well to Dots Per Inch (DPI) on a printer. If you created an image that was 5 Pixels per Inch in Photoshop, and told it to be 3 inches by 3 inches, you would have a 15 pixel by 15 pixel image. Just like on Roll20, if you zoom in or out on that image, the size on screen will change. At 200% zoom, your 15 pixel by 15 pixel image will be 30 pixels x 30 pixels. At 100% zoom, 15 pixels (image) == 3 Inches == 15 pixels (screen). At 200% zoom, 15 pixels (image) == 3 inches == 30 pixels (screen). At 200% zoom each image pixel will be 2 screen pixels, the image will look "pixelated" or "retro". =D Practical usage: When I create Maps or Tokens, I set the Pixels per Inch to either 70 (for maps) or 140 (for tokens). That allows me to easily specify the size in Units of the thing I'm drawing by putting in the dimensions in inches. So, if I want to make a 20x20 map, I put in 70 for the Pixels per Inch and then I put 20 inches wide and 20 inches tall. That is exactly equivalent to just putting in the pixel dimensions of 1400 pixels by 1400 pixels. (If I were to go into the settings for the image and change the Pixels per Inch to 140, my image would say it's size is 10 inches by 10 inches, but the pixel size would still be 1400x1400. Pixel size = (Pixels per Inch x Inch Size), Pixels per Inch = Pixel Size/Inch Size ). Another benefit of doing it this way is that the measure tool will let you measure in whichever measurement scale you like, so setting it to inches lets you measure distances on your map in Units. Hopefully that helps!
