We found this wonderful tutorial by Thread Riding Hood on how to draft a Pinafore pattern pieces in any size. The drafting tutorial required a simple woven dress bodice pattern to use as a base. Thread Riding Tutorial recommends using the First Day Dress as a base.
Take note: this tutorial assumes your base pattern has a 1/2″ seam allowance. If your seam allowance is different, be sure to compensate for that when drafting the pattern.
The Pinafore Bodice is made is 5 steps, starting with tracing the bodice front and back on a blank sheet of papaer, while the pinafore skirt requires 3 large rectangles to be sewn together.
For the full tutorial and measurements, check out Thread Riding Hood.