Sheepskin Cossack hat tutorial with customizable heights. To size you're hat measure around your head (circumference). Use that measurement to choose your pattern. Going off the size you think you are without measuring like medium or small alone could be off since no two company’s mediums will be the same. Going with the max measurement for any given size will give a snug cap, and vise versa.

X-Small- 18.5" to 19.5" (46.9cm to 49.5cm)
Small- 19.5" to 20.5" (49.5cm to 52cm)
Medium- 20.5" to 21.5" (52cm to 54.6cm)
Large- 21.5" to 22.5" (54.6cm to 57cm)
X-Large- 22.5" to 23.5" (57cm to 59.6cm)
XX-Large- 23.5" to 24.5" (59.6cm to 62.2cm)
XXX-Large- 24.5" to 25.5" (62.2cm to 64.7cm)