What we call a centimeter can be however large we choose. Say it were twice as large as it is now and we all agreed that size is a centimeter. What if we chose the inch and the centimeter to be the same size? Agreement doesn't negate the fact that we could have chosen it to be any arbitrary size.

The only true "universals" are universal constants like the speed of light or Planck's Constant. They set boundaries and scales irrespective of the units we use to describe them.