This also works.

She gets a tape measure and tries eight feet, sixteen feet, and thirty-two feet unless one of those fails. 

Eight works; sixteen fails.

Eight is more than enough for something with the function of a purse, since one should ideally be able to reach an arm in and grab anything in there. It's even enough for a suitcase, at least if one is going on vacation rather than moving. If she wants to sell portable houses she might need to draw bigger runes, but that should wait until she's gotten established with smaller boxes and talked to some town planner-type people. 

But as an answer to "how far can I stretch space", "far enough" is more practically than scientifically satisfying. She starts narrowing it down to the nearest six inches, starting with twelve feet and adjusting up or down from there as warranted.

Her size of diagram will do ten feet ten inches.

Nice. Does that number change when she starts with a larger starting box?

Yup. It seems to be a multiplier on the total volume of the space.

That's so nice and logical; good for you, magic physics. The next thing to do is clearly to find out if space-folding objects can be brought into Avalons, but she doesn't want to just try it. After all, for all she knows any special dragon spell-disaster-prevention powers she has only work on spells she cast herself. She goes online and starts researching the government of the Seattle Avalon.

It has an Avalon Council, with a representative from the local Nemean council of elders and otherwise elected by the general Avalon residential population; nonresident critters may purchase the right to vote without moving in if they so desire.

Does there appear to be a channel for contacting a council member, whether open to non-voters or otherwise? How would someone go about opening a storefront or similar?

They haven't got an email form, but they have a mailbox. To get a storefront you'd need to buy or rent property on frontage zoned for that.

She types and erases several drafts of a letter asking to meet and discuss potential space expansion before giving up. She just isn't sure how to explain her level of confidence in her own abilities without either mentioning potential dragon powers or stretching the truth in ways she would object to someone else doing to her. She needs to find another runecaster and pick their brain about what's possible and what isn't, and with that aim in mind she heads to the Avalon again a few days after her previous trip.

There's a magic shop! Medallions, curiosities, luck charms and such.

Ooh, luck charms in person! "How do these work?" She asks the proprietor.

"You wear 'em and they make you luckier," he says.

"Can you control the luck at all? Like, if I wore one and rolled a die a hundred times while wanting twos, would I get lots of twos? Or would it just be things that are generally thought of as good luck for anybody?" 

"Buy one and check," he suggests, sounding bored.

That's reasonable, but so is wanting to know how they work before she buys one. "How much are they? And did you make them?"

"The cheap ones are two hundred bucks and they run up as high as two K," he says. "I make some of 'em."

"I'm a bit surprised you haven't investigated in more detail, if you made them. Do you wear one?"

"Sure do."

"What sort of things have you noticed it doing?"

"Don't have as many close calls in traffic. You gonna buy something or what?"

"I'm actually studying runecasting myself and mostly wanted to talk shop; if you're busy I can leave you alone." She starts examining the other things they have other than medallions and luck charms.

He doesn't look busy but he doesn't seem to want to chat. He watches her look over the merchandise, most of which is behind glass cases and much of which is not clearly labeled. "At least nobody's as obnoxious as that winged lion," he mutters.