Based on the Appraisal text that is displayed when you have an exceptionally large or small Pokémon, it appears that the Pokédex entry is a height/weight reference, giving the average height/weight of that species.

[Here](http://pokemongohub.net/pokemon-go-appraisal-chart/) is a chart with all of the Appraisal quotes.

Note: These are quotes from Blanche, the leader of Team Mystic:

>Phase #3: Pokémon Size Size
>
> - Your [Pokémon Name] is tinier than any we have on record. Astounding.   **XS**
> - Your [Pokémon Name] is below average in size.   **Small**
> - Your [Pokémon Name] is above average in size.   **Large**
> - The size of your [Pokémon Name] is… colossal. This is indeed exceptional. Fascinating!  **XL**

Since height/weight have no bearing on a Pokémon's IVs or CP, these don't really mean anything for your Pokémon other than just being a fun fact.