A paratroop infantry squad of 5 men should be size 1.
You're right that there is no general law. That's because it's very hard to come up with one. What would happen than is that people would argue over it endlessly. And find examples of it being 'unrealistic'. So there's just a general guideline and in the end the decision of the designers for official stats.
It seems to me, that would be excellent, if developers here have expressed and have once and for all put an end in these discussions. It is necessary to clear a situation - how to put the size of a unit and why in one case it is put 0, and in other 1.