This pillow doesn. Determine Your Size. Click Below for Measuring Guide. To properly measure yourself, please print the Custom Measuring Ruler at 100% and cut along the outside black line. Follow the diagram for where to position on the neck to find out what size to order.. Hold the ruler level and measure from the base of the neck to the outside tip of the bony prominence at the shoulder. NOTE: Ignore any muscle mass beyond the bony prominence. Choose one size smaller if you measure between two sizes or sleep on a soft surface such as memory foam or pillow-top mattress.. Pillow Size – Shoulder Measurement. Petite: 4.