As far as prey size if you can't weigh things out just go with something 1-1.5x the widest part of the snake's body. Don't worry about the head or neck size. They can unhinge those jaws and amaze you with the things they can eat. Especially some of the python species.
I also prefer Perfect Prey or Big Cheese for feeders only because Layne Labs shipping is substantially higher to my part of the country.