Call Dragon. This spell seems really powerful compared to other spells of a similar or higher level. How do you keep it in check? What stops pcs from sending it into a dungeon to clear it out for them? There is no guideline on the physical size of the dragon so I’m not sure how large it is supposed to be.
I guess it just really feels like an ‘I win’ button in a way that even 6th level wizard spells don’t. Afterall…there is no Save vs Dragon!
The dragon is almost certainly be too large to fit into a standard dungeon (see below).
The dragon can only be summoned once per week, and it takes 10 minutes to do so. Once summoned the dragon can only stick around for an hour or two.
The dragon can be dispelled with dispel magic with no saving throw. 6th level clerics and 5th level mages can both cast dispel magic. A goblin tribe’s witch doctor or shaman could thus deal with the dragon, as could any evil dark lord of even modest level.
My expectation was that Call Dragon would primarily be used in pitched battles fought outdoors. But if you are finding it’s overly powerful as used in the context of your campaign, then ditch it.
As to your subsidiary question: How big is a dragon? ACKS states that a 1 HD pterodactyl has a 10’ wingspan, while a 5 HD pteranodon has a 25’ wingspan. Let’s therefore assume the formula for calculating wingspan from HD is:
wingpan = HD^(1/2) * 10 feet
Therefore a 14 HD dragon has 38’ wingspan, a 20 HD dragon has a 45’ wingspan, and a 36 HC roc has a 60’ wingspan. (The formula scales down, too: A 1hp bird (.22HD) would have a wingspan of .48’ or about 6".)
The largest known flying creature in real life is Quetzalcoatlus, with a wingspan estimated at 36’. That would make it a 13HD monster in ACKS. It seems to have had a length of about 90% of its wingspan, and a width at the shoulder of about 20% of its wingspan. If a dragon has a similar build to a Quetzalcoatlus, then an Old Dragon would be (45’ x 0.9) 40.5’ long and 9’ wide at the shoulder. Given that dragons are generally depicted as sturdier and better armored than pteranodons and their ilk, it’s likely to be even thicker.