Sorry I can't help with exact stitch sizes cause my machine is metric, but I hear your frustration it's so hard when things are not going right. I don't think the stitch size will affect bowing much I think that's more to with the one of the fabrics getting stretched during sewing, Does your machine have adjustable pressure on the foot, sometimes the pressure on the foot will cause the top fabric to stetch more than the bottom, if you are able to loosen the pressure a bit, this might help. Do you have an even feed or walking foot, using that might help too. Just a couple of ideas, I hope something works for you and that you can get back to enjoying your project :-)