At the shop we've use a simple graphic to show folks why sizing should be for shaft length. Three paddles - overall lengths of 54" (BB Arrow), 57" (BB Beavertail) and 63" (Adirondack Guideboat "sneak" paddle). One picture has them leaning on the car blade down - height difference is plain. Second picture has them leaning up against the car, grip down. Shaft length is clearly the same.
The mantra is: shaft length is for size, blade choice is for use. And yes, that's an oversimplification, as other comments attest, but it's a good starting point for new folks.
If water temp and weather is suitable, we try to get folks into a canoe with a paddle to get best length. We've got a pretty representative demo fleet, so that helps.
If we can't get them in a boat, we generally use the chair method for new paddlers. As described, sit on the hard chair, grip between the legs, measure to the eyebrows for straight shaft. The goal is to end up with a shaft length that puts the grip hand between shoulder and chin, hand along the gunwale and enough shaft to sink the blade into the water. All of the methods are approximate (Grey Owl says +/- 2 inches). We recommend new folks start with a solid, but not expensive, blade that will serve as a good spare if they find there's a better length.
I did find it interesting that when I shifted to kneeling in the chine to get closer to the water, that that small change was enough to make me look for a slightly shorter shaft.