There's a limit of 10 KB for a single key-value, including attributes such as label, content-type, tags, and other metadata. There's no limit on the number of keys and labels as long as their total size is below the storage limit.
This key-value limit should be sufficient for a single setting in most applications. If you find that your setting is larger than this limit, you may consider storing your data elsewhere, and add a reference of that data in App Configuration.
For a complete list of limits, see Azure subscription and service limits.