Whenever I evolve a low CP Pokemon, it becomes XS. Does evolving a high CP Pokemon give it XL weight/height?
I don't have enough candy or stardust to test it myself.
A Pokémon's size and weight are by no means linked to neither the CP nor HP. They do not, as of yet, serve any practical purpose.
XS and XL are completely separate from CP. XS and XL simply effect how much hp a pokemon has and how fast it attacks. XS pokemon attack faster but has lower hp and vice versa for XL. If I'm not mistaken XS and XL are completely random even after evolution.
In the end CP matters the most as you are leveling up. You might start to care about XS vs XL once you get into serious gym battles and everyone is around max CP.
I just evolved a high CP XL Magikarp and it turned into a high CP XS Gyarados. So no CP does not affect size.
Every xl weight evolves into a xs weight and it'll get a cp boost