And now, a glimpse in to how my brain works:

1 - It's definitely canon that this wouldn't shrink or grow clothing. The ability to make things go that well would be a sign of advanced skill and not accidentally making transformation food.

2 - Readers will most likely be expecting, and some rooting for, some manner of, um, "wardrobe situation" being implied.

3 - Rhoda wouldn't be hurt, but clothes are expensive, and Rhoda likes them! I don't want them to get all torn up! That would be dreadful.

4 - Hold on! It should be a somewhat gradual morph! She'd have time to just take them off! Marvelous!

5 - BUT WAIT. Rhoda commonly wears a stylish collar. That'd get snug and break quickly, which is bad for a lot of reasons! Her earrings would be fine because the magic would travel through the piercing and make them grow, too, because that's obviously how it works because OBVIOUSLY, but a brief moment of discomfort around the neck followed by the collar breaking? That's no good!

6 - Ah-HA, but what if she were to be eating the cookie for dessert and was already in pajamas? She wouldn't be wearing jewelry OR the collar! Why, it could even be in the morning! CheckMATE, giant cookie that makes Rhoda grow scenario!

And that's how my brain works.