It's a really basic and straightforward method of sizing the pieces. The concept also applies to any other model you're making (ODST, for instance). effectively, you look at reference pictures to determine reference points that correspond to your body. You then use a measuring tape to measure from point to point on your body to determine how tall each piece needs to be and use that value as the height value for the appropriate pepakura model. For instance, the chest should be measured from where your elbows meet your side to the bottom of your ears. The thigh piece is measured from the joint in your hip to the top of your kneecap. So on, and so forth.
Like I said, it's pretty straightforward and very logical. Abrant3 had a series of tutorials going through more than just sizing, but his sizing tutorial gave you reference points to measure from and the height values that he used for himself. It was INCREDIBLY helpful. The greatest part about it is that it accounts for the infinite variables between each person's body, because each person's body is different - one 6' person's legs are longer than another 6' tall person's legs, for instance. And by measuring off of your own body, you ensure the most accurate fit; you basically tailor-make your armor to fit your body, which is as good as it gets. A scaling value is a quick way to get something close, but not perfect.
If nobody else does so by the time I finish my armor, I'll make a very similar sizing video to the one he took down. That way, people can have the same help I received that made my job so much easier.
