Our general recommendation is that anyone 5'3" to 6'3" tall should order the STANDARD, a person 4' to 5'3" should order a SMALL, and a person 6'4" tall or over should order the LARGE Pillo1. There are certainly exceptions: If a person under 6'4" and is somewhere around 6'2" or 6'3" and is quite muscular or has a large body frame with a long neck and generally weighs over 225 lbs, they would probably enjoy a LARGE size. If a person is around the 5'2" height with a "long" neck, he or she may be better suited for a "STANDARD" size. And finally a person 5'3" with a "short" neck be be best suited for a SMALL size. Through our extensive research on the fitting issue, there are variations to the sizing that we are generally recommending, but by and large, the size charts provided in the order page are very good guidelines for the exact fit for the dimensions of PILLO1. Also, it is very important to understand that the PILLO1 variability in sizes are for body types and dimensions and NOT for pillow sizes such as small, queen, and king sizes like most other beds and pillows. In fact, the length of the standard and large sizes are the same (however the length of the small is less) so don't decide the size of your PILLO1 based on your bed size but only with regard to your body size.