So, dragons are hexapodal creatures, slightly larger than most draft horses (around 190-200 cm at the withers). Compared to the Quetzalcoatlus, dragons have twice the muscles mass in the pectoral region, but the length of the fibers stays consistent. Basically, the chest is just as deep, but the length is extended.
The extra muscle is attached to the heavily modified humerus. Dragon bones are very strong, due to their microscopic structure, which mimics limpet teeth.
That's how dragons remain volant, despite their size. Now, their breath weapon is a two-component acid, aqua regia. Its constituents, nitric acid and hydrochloric acid, are stored separately for safety reasons and only combine moments before exiting the dragon's mouth. Still, having too much acid could interfere with the dragon's ability to fly.
On top of that, the dragon's saliva and mucus are only able to stall (but not completely stop) and (with sodium bicarbonate) neutralize the acids, when present in large-enough quantities.
The breath weapon is useful for dealing with humans and their equipment, but not for much else.
Dragons usually spray targets with the acids, so the two components don't really have a chance to stay combined.
Given that, what's the realistic amount of aqua regia (so, the sum of the two components in liters) a dragon should store?