Generally, there is no true sizing when it comes to sticks due to the high amount of preference that comes with choosing a size of stick. Factors such as stance, play style, and comfortability come into play when choosing the right stick. For a goalie choosing a stick for the first time, we personally recommend getting a stick relative to the rest of your gear sizes. For example, Senior pads, glove, and blocker goalies should use a Senior stick.
As for the specific number sizing of a stick, this is where the other factors roll in to place so, finding another stick as a reference to feel out the size is the best advice if you truly don't know what size you are. A stick should allow the goaltender to comfortably be in a "ready stance" with the stick flush to the ground between their pads.
If you are looking for more in-depth advice on selecting the right goalie stick, please visit Goalie Stick Buying Guide for a more in-depth look at how a goalie stick should fit, or contact our customer support team for personalized guidance.