It's pretty obvious that the reason why Leia used a thermal detonator, and why her use of it put Jabba's court as her mercy, is because the detonation was triggered through a dead-man's switch (release = detonation) OR a ticking timer that had to be deactivated.. or both. The detonator was clearly an analogue with using a hand grenade to the same effect in a present-day Earth situation.
Anyway, I've heard things about thermal detonators having a Class that refers to their yield, with Class A being the most powerful (20 meter radius was it?), and Class E the lowest in yield (2.5 meters or something like that). A while back there was a thread about taking out a Star wars skyscraper with a single Class A TD (something belonging to Xizor I believe). This might be slightly inconsistent with the 20 meter radius idea, although not necessarily so if it destroyed one or more critical support pillars (made of super strong SW material no less). However, I believe the TD was simply tossed down a ventilation shaft or somesuch, not placed precisely, and it was a very large building.
There's also descriptions along the lines of a thermal detonator "a radius damage grenade releasing a barrage of thermal energy capable of disintegrating anything around it", but not saying anything about fixed radius. The term "radius damage" seems to simply indicate that it's not a point-effect weapon. And something about "baradium"?
I have no idea where this information originates though (all second-hand). Wherever it all comes from, is it in the same level of canon as the "fixed radius" idea?
Tbh I hadn't heard of the fixed radius thing till now (as opposed to effects dropping off more or less as per the inverse-square law). As has already been mentioned, it seems improbable for obvious reasons... requiring an explanation
besides the first impression that it simply releases a lot of heat.
Edit:
Darwin wrote:That's right. there'd be damage outside the sphere, and near total destruction inside. would this be inconsistant with text descriptions of the effects?
There wouldn't be a defined border at which the amount of damage suddenly drops off. Even if there was a field that reflected the energy back inward, it would be conserved and would eventually have to go
somewhere when the field dropped.. you'd end up with the same effect of there never having had been a field at all.