The same powers that she possesses.

Link told the Great Fairy that if it was possible, he would appreciate some portion of her own power. His answer set the Great Fairy to laughing even more. "Ohohohohohoho! That is an interesting answer, young hero! Here, take as much as you want!" They she twisted in mid-air, so she could face him directly, and extended both her hands towards him. A bright reddish-pink light beamed out of her hands and into Link's chest. The swordsman half-collapsed to the floor, only supported with one leg, as his body heated up and began to soak up the energy directed towards him. Link didn't understand what was happening, but he felt his body start to shift under his clothes, his waist growing smaller while the top of his tunic grew strained as something expanded under it. He also felt himself get taller, and when he finally felt the magic subside the room looked a little smaller than it had before, once he looked past his red hair. Red hair? Link reached up with one dainty hand and found that his hair was much longer now, and the same bright red as the Great Fairy's. He looked down at himself and saw large breasts tenting out underneath his tunic, with a little bit of cleavage poking out from the top where the tunic couldn't keep them in. His tights felt tight on his womanly rear, and he couldn't feel his metaphorical Master Sword in his pants at all. Shakingly, he walked over to the fountain's edge and looked in the water, staring at the reflection. An exact copy of the Great Fairy's face looked back at him, and Link realized he was now an exact copy of the woman, except from what he could tell he was merely six feet tall, not twelve like her. Unsure of what to do next, Link thought about his options.