Resizing is often more effective than compressing. A 4000-pixel-wide photo published on a website is displayed at 1200 anyway: the other 2800 pixels are weight carried for nothing.
Some practical reference points: 1920 pixels for a full-width image on a website, 1200 for an article or a post, 800 for an attachment to be read on screen, 400 for a thumbnail. For a document to be printed on A4 you need at least 2400 pixels on the long side.
The height is calculated automatically so the image isn't distorted, and you can resize several files at once.