She nods. "That's reasonable, yeah."
Apparently most thing's that're bigger on the inside are sold at a specialty shop - a little store with a blue front, windows plastered with advertisements for different boxes and purses and trunks and wardrobes. The inside dimensions are a bit unrealistic for the storefront, and the walls are stacked with different pieces of furniture. There's a display of trunks, advertising that you can play around with each to find the perfect fit, and a little informative plaque that the interior capacity is independent of the outside design - you can order whichever ornamentation you want.
The displayed trunks are all plain. The cheaper ones have a simple minor space expansion spell on them; the slightly less cheap ones have shelves with separate space expansion spells. The nice ones, which Professor Reynolds steers her to, have different compartments which can only be switched between while the trunk's closed, which can have their own spells and shelving systems applied to them. Compartment numbers range between 'two' and 'nine', with a note that higher capacity trunks must be custom ordered. All of the trunks have featherweight spells applied, to make them easier to move, though there's a warning that you shouldn't cast shrinking charms on the ones with space expansion or multiple compartments.
(The compartmentalized trunks seem to run about fifty galleons per compartment as a base cost, not including further expansion spells, extra protective spells, or high end decoration.)