Listed below is an average helmet general size chart it may not be exact to the novelty helmets but rather an informational guide for you to estimate a size.
Adult
Hat Size
Inches
CM
XSmall
6 5/8 - 6 3/4
20.87 - 21.26
53 - 54
Small
6 7/8 - 7
21.65 - 22.05
55 - 56
Medium
7 1/8 - 7 1/4
22.44 - 22.83
57 - 58
Large
7 3/8 - 7 1/2
23.23 - 23.62
59 - 60
XLarge
7 5/8 - 7 3/4
24.02 - 24.41
61 - 62
2XLarge
7 7/8 - 8
24.80 - 25.20
63 - 64