"Giant" mealworms are treated with hormones and do not make good feeders. Which size to use just depends on your preference and size of the gecko.
The so-called "Giant" Mealworms are created by spraying "common" Mealworms with an insect growth hormone to keep them growing instead of pupating as they would normally. Because of this, we feel that the "Giants" are a poor food choice for your Reptiles. However, we do highly recommend them as fishing bait!