What is How to resize an image (without losing quality)?
TL;DR: To resize an image: pick exact width/height (or a preset), choose 'contain' to keep aspect ratio, and downscale rather than upscale for best quality. Shrink44 does it in-browser.
Resizing changes an image's pixel dimensions. Downscaling (making it smaller) keeps excellent quality; upscaling beyond the original resolution can't add real detail.
Decide the target size by use: ~1920px for desktop web, ~1080px for mobile, social presets for each platform, or up to 4K (3840px) for large displays.