What is the size of a sheath of bladestealth?

[Dark Sorcerer]

Does the sheath of bladestealth change its own size or does it change the size of the weapon?

This mundane-looking but serviceable sheath fits any bladed slashing weapon, from a dagger to a greatsword. When a weapon is placed into the sheath of bladestealth, both it and the weapon become invisible (as the invisibility spell) until the weapon is drawn, at which point it and the sheath become visible again. The invisible sheathed weapon can still be detected by touch, but the bearer gains a +5 bonus on Sleight of Hand checks to conceal the weapon, and casual inspection does not reveal it at all. Other effects that detect or reveal invisible things work normally on the sheathed weapon.

[Speaker for the Dead]

Since the text you quoted is worded, "sheath fits any bladed slashing weapon" it sounds like the sheath changes to fit the blade.

[lemeres]

The fact that the weapon can be revealed by touch also might be a clue to the sheath changing size.

If it was just a dagger sized sheath that swallowed up your greatsword, there would be little chance that someone would accidentally bump into it.

Really, if I was making an invisible container that could hide huge weapons, I would be tempted not to shape it like a sheath at all. Make it some small bag so that no one would be able to guess it was a weapon if they frisked you (admittedly they might think it was just a dagger, but it would still tip people off)