For chest circumference, measure the circumference of your upper body across the widest part of your bust. Make sure the measuring tape is horizontal (and not crooked) and that the bust is not pressed down.
Measure the sleeve length from the tip of your shoulder to the bottom of your wrist.
The overall desired length of the gown is the distance from the top of your shoulder to the bottom of the gown. Fifteen inches from the ground is a common length.