How to to design a PCB lib for components with variable dimensions?
Just assume a worst-case scenario for each dimension (usually that means the largest dim), then you'll be safe as long as the manufacturer stays within their reported tolerance.

As Jack B mentioned in a comment, the example you gave is for standard resistor sizes so it's a poor example. But, assuming there wasn't a library for them in whatever software you use, you could design the 0603 footprint as 1.7x0.9x0.5