Have her growth tied to specific conditions with accompanying mental changes.

Tom had always been a fan of powerful giantesses who were paradoxically submissive and dependant on their little lover for decisions and validation. With that, he resolved to slowly but surely turn Sarah into an willingly obedient and affectionate sex-slave with the size and physical power of a kaiju-goddess.

Taking a moment to write out his thoughts and edit the wording of his wish, he took up the coin.

Rubbing the coin, Tom whispered: "I wish that from this time forward Sarah would grow taller, stronger, more physically resilient, but also more submissive, loyal, and willingly obedient to me specifically, each time she takes any of my cum into her body. These improvements occur at a rate of +5% for every occasion which I cum into her anus or vagina, +10% for each time she consumes my cum orally, an additional +15% for every occasion in which she meets two of these three conditions in a 1-hour period, and additional +25% for each occasion where she meets all three conditions in a 1-hour period. All of the aforementioned factors will be doubled for all occasions where Sarah orgasms within 5 minutes after consuming my cum.