Every length are scaled based on what's called the rem (Root Emphemeral Unit) which is equal by default to 16px.
This scale is used for font size and spacing (component spacing and line spacing)
The numerical scale is going:
- from 1 for a length value of .25rem
- to 5 for a length value of 3rem
The alphabetical scale is going:
- from xs (extra-small) for a length value of 1rem
- to lg (large) for a length value of 2rem
Every browser sets a value of 16px for one rem by default but you can increase it with the remSize configuration.
This value is not dependent on the screen size (Bootstrap does not vary it either)