Sizing of military uniforms before the later parts of the 19th Century was a relatively simple one. In part it either fit you "off the rack" in generally three sizes, or addition to that was custom tailored by in-house by members of the individual units. This practice has been well documented in American, French and British Armies of the period, and beyond.
Cocked hats appear to be no different. Though the evidence is somewhat circumstancial, an order concerning the hats of the Britsh Army in the late 1780's stipulated that "hatts [sic] are to be in the future, no smaller than 7 inches round and four inches deep in the crown".
Further evidence for sizing comes from the civilian world of this era. A 1789 book, entitled Instructions For Cutting Out Apparel For the Poor (availible for free from google books) wrote that clothing for men was generally ready made in "slop shops" in a few sizes. Further the annoynmous author states that Hats were availble for men in three sizes, ranging from no. 1 (the largest @ 1s. 10p) to no. 3 (smallest @ 1s. 1p.)