Depends what the use is for - generally, error on the side of shorter. If he is going to use where there is a little climbing involved, go with a slightly more technical tool as the BD Venom. You can always double up for a more technical setup down the road.

Generally, there is not "set" size as body types differ - how you measure is boots on - it should be approximately the length between the tips of your fingers and the floor when standing with your arms at your sides - measured in centimeters. Some people like to test this with a stick cut to length before buying to make sure they are comfy with the length.

Remember too - there are options from Petzl that double as an adjustable "trekking pole" depending on use.