Is it more powerful to cast a wiccan spell on a full moon?

That would depend entirely on the spell you are casting.

- Spells of completion, harvest and thanks are best cast at the Full moon.

- Spells of beginning, planting, and growth are best cast at the New moon.

- Spells of increase are best cast during a waxing moon (from the New moon to the Full moon)

- Spells of decrease are best cast during a waning moon (from the Full moon to the New moon)

- Specific spells cast on a Blue moon (the second full moon in a calendar month) can be extremely powerful. This is an Excellent time to charge talismans and amulets.

- Specific spell cast on a Black moon (the second New moon of a calendar month) can be extremely powerful. This is an Excellent time to cast major spells of change or beginning.

The house the moon is in will also have considerable effect on the spell. It would do little good to do a prosperity spell in a waning moon in Libra, whereas casting it in a waxing moon in Taurus would make it much stronger.

There is also the whole "void of coarse" thing. The time between when the moon leaves one house and enters another. Spells cast during this time tend to be less effective and erratic.