When I started to create my first morph, Amy for Victoria 3, I questioned about that.  In fact, I don't think there is an universal answer.  I noticed DAZ is based on 1unit = 8ft as the scale of their products, it is reflected in DAZ Studio.  At that time, I made a metre stick (I'm living in a country using metric system), I used it to measure the height of the characters.  Therefore, I found that Victoria is taller than me, Aiko is shorter than me.  At the end, I created a scale pose for my morph. Amy for Victoria 3/Aiko 3/Victoria 4.2 is 170cm in height.  Then, I found that Dr. Geep has a good tutorial about the scale.  As long as you stick to one scale, you should be fine.  If you get some new props, you can use your own measurement to re-scale it.  For example, when you get the model of the house, you can measure the height of the door and use it as a reference.  Based on the fact that constructions in real world follow some standards, you can figure out how tall is that door should be.  It works for me.  I'm sure that someone has another way.