Dude, a flash on-camera will cover the subjects at all but the extreme focal lengths.
Anything you get larger will appear softer. But the farther away you get it the harder your little flash will have to work. There's no way I'd use a 60 inch anything with a speedlight....and while some here DO use large modifiers, they know the flash and it's capabilities. If you're asking this question, it means you're new to this type of thing. Don't go big. AND...keep your shutter speed at or below the synch speed of your camera when you use modifiers to keep from losing a ton of power in your flashes.