Computing - binary style!
The basis behind all modern computing lies in bits. A specific bit can have one of two states: on and off, or 1 and 0, but not both.
Computing - quantum style!
We haven't figured this out yet, but the abstract idea is that there's a lot of room between on and off, or 1 and 0. We enable the use of all of these middle states and can improve computing power as well as memory capacity at once. Again, have we yet? No, most qubit-using computers (qubits are the quantum version of bits) are very expensive.
CRISPR Gene Editing
From wikipedia: CRISPR gene editing is a method by which the genomes of living organisms may be edited. It is based on a simplified version of the bacterial CRISPR/Cas antiviral defense system.
The power your character has is to basically initiate CRISPR-style gene editing on a cat or something and manipulate it. If we take the supposedly simple idea of the cat being big or small, we realize that there's a lot of room in between. So, your character imagines the specific features of this cat that need to be bigger as well as the features that don't need to change. This is done by the character using their power to apply CRISPR gene editing on every cell of the animal's body using the elements and compounds found within the animal itself. Notice, I said all because if we only manipulated some, they could be seen as foreign cells by the cat and eliminated by the pre-existing body as if they were a problem, which is not what you want. How does the character have this power? I have no idea, but I feel like that's another question to answer.
Edit: I realized I forgot to mention something else. Since basic quantum computing is arguably simpler than changing the DNA in every cell in a person's body simultaneously, and because we still don't have a solution to quantum computing, this allows for a character to slowly build these skills over time, but with horrifying consequences. In my mind at least, you can't go a 'little wrong' with this kind of power. Either you did what you had in mind, or you accidentally turned what used to be a cat into a mess of organic material.