How big is big enough? What's the requirements on where a Rune Circle is placed?
Regardless, I think we can get it bigger.
Here's a quote from the "Creating a Rune Circle" section. I don't think there's a minimum size for how big place has to be where the Rune Circle is made...
Races of Stone, p.167
Creating A Rune Circle"A rune circle is created by fusing magic runes into the ground or into the floor of a chamber. Once it is placed, a rune circle cannot be moved. Moving the stone that a rune circle is engraved upon always ruins the rune circle. Because the creation process creates a threefold connection between the location, the magic energy expended in the creation, and the physical earth or stone in which the runes are drawn, there’s no way to create a mobile rune circle. Even vehicles large enough to contain an entire rune circle do not allow for the creation of a mobile rune circle."
I think the line "there’s no way to create a mobile rune circle" pretty much kills the subject. Still ... a flying castle? Maybe.