Bernadotte,
Unfortunately it is something of a guessing game as to how far an army HQ's command radius is. I guess with practice one gets the hang of how far command radius extends for each army HQ's leader. I think that the command radius being much smaller than the attachment radius is realistic. Remember that communications between army and corp commanders was reliant upon how fast horse-borne couriers could make the trip or even if they could make the trip without being intercepted.
Good army leaders like Napoleon seem to go at most two regions away but the poor army leaders seem to only have a range of the same region. I find that with the really poor army leaders, like the Prussian ones in 1806, it's better to move them way out of command range so they don't pass along negative ratings to their corps.
It certainly would be a nice player aid if there was a function to show us command radius for each army leader the same way that the one that shows us attachment radius. Hopefully that might come about in future releases like NCP2.