This is the Philadelphia Phillies' 2012 Authentic alternate home on-field cap made by New Era. The Phillies wear this cap for all home day games. The club introduced this style in 2008; it is inspired by the caps worn by the Phillies from 1946 to 1949. 

Cap is Authentic Collection 59FIFTY® which is the official on-field cap of Major League Baseball® and is worn by every Atlanta player. Made of 100% polyester, and designed with an embroidered (raised) Atlanta team logo on front and stitched Major League Baseball® logo on the rear. Interior includes branded taping as well as a CoolBase moisture wicking black sweatband. Fitted, closed back.

If you do not know your size: Determine where you want to wear your hat. Usually it is worn about an inch above the eyebrows and/or just above the ears. Have a friend hold the tape measure (cloth or plastic) at the point where you want the hat to rest on your head.  Insert the tip of your little finger between the tape and your head.  Record the measurement.  This should provide you with a comfortable fit.