I haven't worked with zones, but I'd guess that it depends on the shape. Squares and rectangles do not have a radius to speak of, so, you know....

Ah... it was my understanding that all zone primatives were pinned at thier centre so radial values would work for the "BOX" cuboid shape too. I think I've figured it out now though anyway from examining the distance of a docking ring from it's planet. it looks like the values are radial. I didn't bring my notebook into the office today and was trying to make some edits to the Alaska system for my mod without being able to test them. I'll just have to test it tonight. If I'm wrong though everything's gonna be too small.

Moose out.

[zone
nickname = Zone_my01_001_Planet_my01_01_atmosphere_burn
POs = -20000, 0, 0 (center of zone)
shape = SPHERE
size = 4600 (distance from center to edge, dunno about box, 10k units = 9km)
edge_fraction = 0.070000
spacedust = atmosphere_gray
spacedust_maxparticles = 250
sort = 99

A box zone must have separate values for its height, width, and length (or half-length, I suppose). Otherwise, they'd all be cubes, right?