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.
Generally what you want to do is to make your logo in a vector program such as Illustrator so that you can scale it up to any size you want. If you prefer to use Photoshop though what I typically do is set it to a size that is fairly large (say 2000x1500) then zoom out to 50% so that it's comfortable to work at it on your screen.