What are the differences between HTML5 and (pre-5) HTML? How can I make HTML-like forms in Markdown? What is difference between Hypertext and Markup in HTML?
You set the inches and the number of pixels in each of those inches. Photoshop removes or adds data to fit the number of pixels in each of the inches you specified. Note: The original image used in the examples below is 4x4 inches, 100 ppi, 400x400 pixels at 468.8 KB.
If you draw in Photoshop it is important to plan ahead for final output size. If you want to eventually print your drawing at 16 x 20, then you need to start with a document that big, at a resolution of at least 150 ppi (depending on how it will be printed). When you draw in Illustrator you don't have to worry about any of that in the beginning.