Essentially AlcheME's Inequivalent Exchange.

Able to grow/shrink himself, objects, and other people, does not change the strength/make up of the item or person, just size-changing. Cannot change parts of people/items, it's wholesale shrink/grow. Can be canceled via other blessings, and wears off over time if he doesn't cancel it himself. The biggest he can go on the boat is double his size?

Absolutely uses this to make himself look taller than he actually is.