For Hats:
Size guide preview (Imperial)
  A B C D
One Size 23 ⅝-24 ⅜ 4 ¾ 2 ¾ 7 ½
Size guide preview (Metric)
  A B C D
One Size 60-62 12 7 19

 

 

For T-shirts:
Size guide preview (Imperial)
Length
Width
Chest
XS
27
16 ½
31-34
S
28
18
34-37
M
29
20
38-41
L
30
22
42-45
XL
31
24
46-49
2XL
32
26
50-53
Size guide preview (Metric)
Length
Width
Chest
XS
68.6
42
78.7-86.4
S
71.1
45.7
86.4-94
M
73.7
50.8
96.5-104.1
L
76.2
55.9
106.7-114.3
XL
78.7
61
116.8-124.5
2XL
81.3
66
127-134.6