Dimensions are the setting people change most often and understand least. "Make it smaller" can mean crop, resize or compress; enlarging a small image invents pixels that were never captured; and the same photo needs wildly different pixel counts for a thumbnail, a print and a retina screen.

These articles cover the whole size question: converting between millimetres, inches and pixels, what DPI actually means, how much resolution each output really needs, the interpolation maths behind scaling, and why an image can look blurry after a resize that should have been harmless. They also cover serving the right size to each device, so a phone never downloads a desktop-sized file.