TBH I didn't give it much thought. It was about shoulder width apart. If I did it again, I'd probably make it narrower and longer as I think it would reduce fatigue and longer strokes would reduce the number of strokes required.
I got the oak at almost those sizes already so no chicken and egg three Richard ;-). It was quite heath robinson, but very effective.
I've just realised that you probably cant see from those photos, but the threaded rod goes right through the end rails. There is a large washer and nut on the other side. Tighten the nuts at either end to bring the blade into tension.
As I mentioned the end rails were not man enough for the job, but those lumps of pallet wood certainly were. Could get a very nice 'ping' on the blade when plucked.