Definitely JPG, as it’s the most ubiquitous file format out there that everyone can view without needing special software.
Recently I’ve been delivering files at 3300px long on the long side, which I think is a good balance between high resolution and smaller file sizes. (3300px is big enough to print at 11 inches long, a standard paper size.)
Check out these resources for lots of details:
https://www.lenspiration.com/forums/topic/what-size-photo-should-i-deliver-to-my-clients/
https://www.lenspiration.com/2020/07/what-quality-setting-should-i-use-for-jpg-photos/