It's a trap. Units are pretty much what you say they are. Props and figures over the years have all used different standards, and scaling is all over the map.

My technique relies on Bob. Bob is a M4. I take Bob, morph him around to resemble a reasonable human being, and then declare: "Bob is six feet tall. Everybody's height and scaling revolves around Bob. This includes props as well as characters. There are none more 72 inches than Bob." Bob gets stashed in a library.

So whenever I need to scale stuff I'll swing Bob in and use the Mk I eyeball. OK, OK, I might cheat a little and use some sizing apparatus thing, but it's all based on Bob.

Remember the Golden Rule of Poser Scaling: if it looks good, it is good.