macOS Creating an empty mp3 or jpg of a specific size

What do you mean by "empty"? For example you could create a huge jpeg (say converted from a raw image) but then manually set the width and height via a hex editor to 0. But whatever program you're using might check for this and classify it as an invalid image.

Can you be more specific on what you're trying to test?

You could just create a NSImage, set its rectangle, and write its bitmap imagerep to a file as a jpeg (if you know how to use Cocoa).

250MB/500MB/or 1000MB is not empty. Do you need to create the file pragmatically, or just create them somehow to use as test data to test a program?

For the JPG, I would create a huge file new image in photoshop or a similar image editor, apply some noise filter so it won't compress easily, then set the dimensions of the image until I got the desired file size.

For the mp3, maybe just start recording in garage band and let is sit overnight? Or create a file with one of the included loops that is 24 or 48 hours long? Not sure as I don't use garage band.