It's a question I hear a lot when teaching basic kayaking. Choose one you think is right. At first, learning to make your kayak steer, moving where you want, launching and landing are much more important than choosing the perfect paddle. Once you've kayaked, perhaps even tried some different types of paddles, you'll be ready to make an educated choice.
Paddles are a personal preference.
There is no right and wrong when choosing. Lots of opinions, theories, and formulas exist, but when it comes down to it, choose one that feels right. Well you probably haven't found the right paddle yet. In an ideal world, each kayaker would use a different paddle for each boat that they paddled. I base the overall length of a paddle on what boat I'm going to be paddling.