Enough people asked about this, and I'm playing this storyline a little bit (not entirely) by ear anyway, so here we are.
Really, Rhoda would have a hard time precisely growing or shrinking something while she was wearing it under ideal conditions, let alone with a time limit. She could do it, though it might wind up a bit bigger or smaller than intended.
Someone did basically ask me about Rhoda's instincts kicking in (as they did with the boar) and her growing the collar on reflex if it became tight around her neck, which was a really good point.
In practice, it would never reach that point, because the growth effect would break the collar before it ever got that tight. If she were somehow in a dangerous non-growing situation in which something was squeezing part of her, however, it is likely she'd grow that thing looser, or shrink herself smaller, or something by way of pure adrenaline saying "do something right the heck now to survive, darn it".
And yes, a rule of thumb is that anything under one's skin (even if just partially) will grow / shrink with the person. Piercings, teeth fillings, cybernetic implants, whatever, would be fine.