Here are some examples:
* Full-length mirror: This is hyphenated because it's used as a compound noun.
* She wore a full-length gown. This is hyphenated because it's used as a compound adjective.
* The movie was a full length film. This is not hyphenated because it's used as an adjective phrase.
Key Takeaway: Hyphenate "full length" when it acts as a compound adjective, but not when it acts as a simple adjective phrase.