Byleth sighs lightly as she looks at the stacks of books and snacks piled against the wall thinking 'Just how long were you planning on staying in here Berni?' as she had also seen the new, reinforced door blocking the archer from the outside world.
Then something on the floor catches her eye so she slips into a kneeling position and picks it up "shrinking potion" she reads, wondering where Bernidetta got it, as well as if it were real or just a prank by the Golden Deer, taking advantage of the poor girls borderline agoraphobia 'maybe it was a laxative and she's in the bathroom, explains the unlocked door' the green haired girl thinks, until she turns her head and spots and tiny Bernidetta staring at her frozen like a statue in the lap of a stuffed bear. The professor's quiet "Berni" seemed to snap her out of the funk as the tiny purplette makes a break for it, only to be easily captured by Byleths quick reflexes.
"Let me go human, I want to be set free!" Bernidetta squeaks, hoping to confuse the giant into thinking she is an actual borrower, but given Byleths unimpressed face it didn't work "Um, hi professor, nice weather were having"
"Bernidetta, why are you small?"
"Magic?"
"I mean besides magic, why did you shrink yourself?"
"I want to be left alone, if I'm tiny people won't see me, and I'll be able to have my quiet alone time" Bernidetta huffs, causing Byleth to roll her eyes
"Bernidetta, this is extreme, even for you. How did you even do this?"
"I bought it from a red headed woman in the marketplace, it was a pretty good price too, just 200 gold" Byleth pinches the bridge of her nose, of course she did.
What next?
1) Byleth, with an unhappy and unwilling Bernidetta accompanying her, heads to the marketplace to find a reversal potion and restore the archer
2) it's decided that if she's determined to do this, at least Edelgard needs to be informed (given Byleth already knows), as the head of Black Eagles. Berni is very nervous about Edelgards reaction, despite Byleth assuring her it will be okay.
3) Byleth doesn't trust Bernidetta to be able to keep herself safe in her new size, do tells her that she will be staying with the professor in her room while she's this size.