Best bet to last 3 years.
It will be large for the majority of 6 year olds, but will have room for a sweater/jacket and will allow for growth over 3 years.
Also gives sufficient space for patches.
We sell this size to about 60% of customers who try on our samples.
(Tandy labels this size as "Small Adult 4650-01")
37" around - 22" long