Its total length is 69cm. Allow at least 5cm for overlap of the velcro, leaving you 64cm.
As viper said, since there is no elastic, it does not stay on as well as it could if it had that simple improvement. It wouldn't be a difficult mod, though [just use pop rivets] and that would also give you extra length.
Using single cell AA or 123 lights, it is very stable for most tasks. I wouldn't expect it to stay while running or biking though. I use 2AA lights in it occasionally - it's ok, but a bit unstable.