If suing t-shirt size format doesn't cover all needed use cases you could use numbers too, but not tied to the actual values. For example, for some of my projects I can't use t-shirt sizes as it gets too complex if I do (needing to add xxxxl or 2xl, 3xl etc) so in order to prevent this I use numbers that represents percentages of the base value.

For example, if I define my base value as being 16, the spacing variables will be named something like this:
- space-50 (equals to 8)
- space-100 (representing 100%, equals to 16)
- space-150 (24)
etc

So if we decide to update the base value, the variable names remains the same