The word for "grow" (increase in size) is Qa. But for animals or plants growing, that is, maturing (not just increasing in size), use nenchoH.

QaHa' would refer to getting smaller after the thing had previously grown bigger — that is, it's losing (in length or whatever) what it had gained. So if the balloon is losing air, you could say it QaHa'. Presumably the object is going back towards it's original size (though it may not get that far or may become even smaller than it once was).

For "get smaller, shrink" (without the notion of having previously grown), use qer. Of course, depending on exactly what you want to say, you can also use machchoH.

The choice depends on whether you're focusing on just the shrinking or growing, on the one hand, or on the return to or towards the original size (or beyond!).

So if your main character started at their original size, and then shrank, that's qer. If they then started to grow, that would be Qa if the storyteller is just saying that they grew, but it would be qerHa' if the storyteller was saying that the shrinking was reversing. If the de-shrinking continued so that the character ended up bigger than they originally were, that might be qerHa' and then — what a surprise! — Qa.

For Pinocchio, if you're saying that after his nose grew he started telling the truth and his nose got smaller, that's probably QaHa'. If you'd just met the long-nosed Pinocchio for the first time and didn't know the story of the nose but saw it get smaller, you'd probably say qer.

In short, you have four words to choose from, depending on what exactly you want to say: qer, qerHa', Qa, QaHa'.