Panel PCs range from 7 to 24 inches, in 4:3 and 16:9. How do we pick the size and aspect ratio?

Start from the software interface, not from the screen size. Aspect ratio: 1. Legacy HMI screens designed for 4:3 or 5:4 (resolutions such as 1024×768 or 1280×1024): keep the old ratio to avoid stretching or black bars, especially when replacing an old unit. 2. New interfaces, chart-heavy dashboards, camera video: 16:9 or 16:10. Size by viewing distance and operation: 7–10 inches: machine-mounted panels, operator standing close, a few large buttons. 12–15 inches: the most common HMI station, viewed at about 50–80 cm. 17–24 inches: information-dense monitoring, multiple cameras, viewed from further away. Also check: 1. On-screen buttons should be large enough for gloved fingers, typically about 10–15 mm or more. 2. The cabinet cutout size and rear depth. 3. Resolution: a large screen with low resolution looks grainy up close. Many modular Panel PC lines let you change screen size while keeping the same computer module.