The Epic Level Handbook includes dragons that are larger than Colossal, having the size "Colossal+".
The rules for dragons of such size as described as follows:
Although there is no size category larger than Colossal, the oldest epic dragons deal more damage with their attacks than other Colossal dragons, as shown on the Epic Dragon Face and Reach and Epic Dragon Attacks tables below. In addition, the breath weapon of the oldest epic dragons is a larger cone than most Colossal dragons possess. The size modifier for these dragons remains -8.
The size modifier, space, and reach of a Colossal+ creature remain the same as for a Colossal creature, but their damage rolls go up by one step, as seen in the table at the top here.