On the numbers part. Maybe we can even get to a common point here.
When we measure something, the first number on a ruler is "0". We start at point 0. When something is 4 centimeters long, we started at 0. When I used to teach primary school the kids got "centimeter cubes" which were each 1 cm. long. They stuck together their centimeter cubes and visually got to see how many centimeters long something was. How many centimeters fit. This is the best image I could find:
This is NOT what our interval names reflect. They don't tell us "how long the length of the interval is". For that, our equivalent to centimeter cubes might be semitones.
Instead, we've got something quite primitive. You're at a row of trees. How far are you supposed to stretch that rope? "Up to the 3rd tree". You're counting your trees: 1st tree, 2nd three, 3rd tree. You start at 1, not 0. That's what the interval names do.
Supposing your trees are 1 yard apart (they're small trees). The distance from the 1st tree to the 3rd tree would be 2 yards. We've got a distance of two yards, but we've gone to the third tree.
The "length" of an interval - or "size" of an interval - is done in a primitive way. "It's over yonder, 4th gopher hole over." and if you stretch your rope first the 1st to the 4th, you get the right length of your rope - this is also a way of "measuring" but not how we think of measuring.