Compress an image to a specific width?

Not that I know of. The file target in Feeds can currently only download the image from a url. So without custom code or additional modules the image would need to be downsized at the source.

I do have written an alternative file target that accepts a base64 string as input in my Feeds DEV → sandbox project. But then you would still need to write Tamper plugins to download the image, downsize it and convert it to a base64 string.

It would probably be better idea to make a change to the image target in Feeds to respect the maximum image dimension setting. Too bad that setting is not evaluated automatically when storing images programmatically.