Free tools
4 image resizers for the networks you post to, at the sizes each one actually shows. They run in your browser, so nothing is uploaded and there is no watermark.
Instagram
Instagram Image Resizer
Any photo to 1080 x 1440, 1080 x 1350, square or story size, cropped where you choose or fitted whole.
Facebook
Facebook Image Resizer
Feed posts, link images, stories and the page cover, at the sizes Facebook actually shows them.
X
Twitter (X) Image Resizer
Landscape posts, square and the profile header for X, checked against its 5 MB image limit.
YouTube
YouTube Thumbnail Resizer
Any image to a 16:9 thumbnail at 1280 x 720 or 4K, or a 9:16 Shorts cover, under YouTube's 2 MB limit.
The size guides behind the tools
Every size the resizers offer is argued for, with sources, in one of these.