How do I know what length to get?

Actually the proper way to size an ice axe is to hold it in either hand, relax your shoulders and stand up straight, and letting it hang from your hand, the bottom spike should be reach down to around your ankle.

Wrong (well, sort of)... An ice axe is more than a walking cane, although people do use it that way - when a trekking pole would probably serve better. The critical use for an ice axe in general mountaineering is in self-arrest. The typical answer for a mountaineering axe is a length when grasped from the head to no longer than to your ankle bone. I am 5-11 and have a 65cm Raven Pro that comes to my ankle. It's been fine for me on stuff like Rainier, but is a bit unwieldly for steeper hard snow and mixed terrain. The steeper the slope and more technical its use, the shorter it should be. Imagine crossing a steep traverse with it to the uphill side, or placing it in front as you climb. If your use is snow walking on relatively gentle slopes, particularly on soft snow, a longer axe will do, although it may actually be more difficult to use in self arrest. The Venom is intended as a crossover for somewhat more technical uses on steeper slopes. That is why it is available only in shorter lengths of 50cm to 64cm. If the intended use is not particularly technical (i.e. actually using the pick in front of you as a tool to climb the steeps), you should maybe consider something lighter. If you look at say the BD Raven Pro, you will find it actually comes in lengths that will reach the floor if that is what you want, but is much lighter. Check out this link for awesome advice. http://www.outdoorgearlab.com/Ice-Axe-Reviews/buying-advice

go to a shop and hold an axe in your dominante hand the axe should hang about an 1/2 to an inch from level ground.