Config Vars are limited by 32KB per application. The Platform will prevent you from attempting to store more than 32KB, but it can sometimes be helpful to know how much space is being used.
You can calculate the size of your Config Vars via the CLI command:
$ heroku config --json -a APP_NAME | tr -d '[:space:]' | wc -c 13245
Here, the Config Vars are using ~13KB of space.