A Pokémon with an XS tag, for either weight or height, is a sign that this Pokémon has lower than average values for those stats. As was explored in this question, it appears that higher weight leads to higher health for two Pokémon of the same CP value. From this, one can conclude that a Pokémon with XL tags for height and weight is generally going to be a better choice than the same Pokémon with XS tags.

I've read in a few places that these tags also influence your Pokémon's moveset, although I haven't seen any evidence to back this up.

A Pokémon with an XS tag, for either weight or height, is a sign that this Pokémon has lower than average values for those stats. As was explored in this question, it appears that higher weight leads to higher health for two Pokémon of the same CP value. From this, one can conclude that a Pokémon with XL tags for height and weight is generally going to be a better choice than the same Pokémon with XS tags.

I've read in a few places that these tags also influence your Pokémon's moveset, although I haven't seen any evidence to back this up.