The Landyachtz 36" Switchblade Jackalope
Skating is a physical form of personal expression, and choosing a longboard should be based in your own enjoyment. Skating is as creative as it is physical. Skaters ride terrain the same way dancers move to sound that inspires them. It's true that setting up a chart and saying "if you're this tall choose this size" can play a part in what you skate, but it should not be the only deciding factor. Make sure the length you choose can perform in the style you want. For this article, I am not factoring in flex, concave, or other features that add different dynamics to a longboard. I am only focusing on length and width.
Length:
Length is subjective. What I consider long may seem short to you. Use your noggin to find what feels comfortable to you.
A longer board usually has bigger characteristics, like a larger turning radius, longer platform for dancing, and more stability. For those humans who take note on wheelbase, a longer board generally means a bigger wheelbase. If you have a wider stance, a lengthy board will provide the space you need.
The Globe 26" Blazer Island Blue
A shorter deck generally has smaller characteristics, like a tighter turn radius and shorter wheelbase. They're easier to maneuver or stash under a desk due to their compact size. If you sport a narrow stance, a shorter length will remove the excess space.
Width:
Again, width is subjective. My idea of a narrow platform may not be the same as yours. Base your choice on the style you are interested in.
A wider the platform will be more stable, but a narrow board will give you more control.
Body type is only one part of a big picture. Instead of using a scientific explanation about weight and height and converting it into a size, consider your personal preference. Base your choice in the characteristics of a size that cater to you and the style you want.