A general rule for Jo length is floor to armpit, standing barefoot. Notice that the measurement is made with the arm in a normal relaxed position.
As always in choosing a wooden weapon, the most important first step for a newcomer is to find out what is preferred in the their dojo. In choosing your jo, all situations are different but we would suggest that customizations in size be generally conservative. For example, a taller practitioner might order a jo up to 53 or 54" jo and a shorter person might simply stick to the original parameters of the shinto muso ryu rather than ordering a very short jo. Excessively long or custom made extra thick jo, unless expressly required by an instructor, will certainly not be welcomed in most weapons practice.