Size of images

How large an image should be depends very much on the application and project.
In general, the “bigger is better” rule applies.

However, the file size should be within limits (< 2 MB).
It is best to run the photo through an image optimizer. This can be done online at tinypng.com.
This way you can downscale the photo to good file size. 2000 px width is more than large enough.