Is there a way to sort photos by file size in iPhoto?
No, there is no way to sort on size in iPhoto. From your context I guess you mean filesize. In iPhoto, size means the dimensions on the Photo (length by breadth, measured in pixels) - see the Export dialogue, for instance. iPhoto is all about the images and not the files that contain them, so I'm not surprised. It's a $15 app, you get a lot, but not everything.