How to choose the correct boot size for snowboarding
Your toes should touch the tips of the snowboard boots, but your toes should not be scrunched up/your toes should not be bent.
The reason is the boots will fill out after 4-8 days of snowboarding. So you do not want boots which feel loose when you're purchasing them.
Most boots can last for 60-70 days of snowboarding before it becomes too loose.