i'm a noob but i might know! i went to a nice yarn shop yest. and asked the same question-- the very helpful person told me that 16" is perfect. you can make bigger stuff than the needles but it's a challenge to make things smaller than the circs, but a 16" for a hat will do well.Â
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you also want the hat to be kind of small and it will stretch. do you use ravelry at all? i found some super easy patterns for a man's hat there, bc i am still learning about gauge and casting on.
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the first time i made a hat for dh i tried to do maths by measuring his head and it was stupid big. i had to get rid of it. it will also look smaller when you're starting on the needles than seems will fit but it will stretch....