Also remember that the longest distance you've ever traveled is an unknown quantity but guaranteed to be huge, since the earth itself is careening through space
So a box means that the duration of time doesn't affect the lengths we can perceive, and the sizes don't affect the duration of time we can observe something.
A circle would imply that the amount of time we perceive something affects the scales we can perceive, and most weirdly of all, vice-versa.
The circle sounds like a really interesting—if mind-bending—science fiction book. If it was written well enough I'd give it a shot, even if I can't intuit what's going on.