yo-lucas-yo asks: Do we have an idea of what Big Red (what I'm calling the Larger - than - Ancient Dragon) would fit under in terms of age and size?
In order to qualify as an Ancient, a dragon must be of at least 801 years of age and Gargantuan (bigger than Huge) in size. By this definition, Big Red would still technically be considered an Ancient Dragon. It is implied that his size is larger than that of the run-of-the-mill ancient red dragon (which are already intimidating as they are), and presumably its age is also notably older than is typical.
Ancient dragons can live for over a thousand years. It is not specified how much more beyond 1000 years until old age is more likely to kill a dragon instead of combat, but it would not be unreasonable to estimate that any four of the members of the Chroma Conclave are over 1500 years in age.
Once a creature hits Gargantuan, it’s hard to go much bigger than that. Mechanically, this deviation from the normal red is probably only reflected by an increased AC, as shown by Big Green’s extra point. Story-wise, its increased size is likely to show its dominance to any other ancient dragon (of any color), as well as provide a significantly more ominous tone. This wasn’t just any dragon, which are dangerous on their own. This was the single most dangerous dragon in existence.
Tl;dr: Gargantuan, and over 1500 years. Maybe.
Update: mashtar points out that in earlier versions of D&D, there was also the Colossal size category (creatures greater than 64 feet in height or length). To put things in perspective, the ridiculously large tarrasque is 50 feet tall, making it both Gargantuan and the largest creature in the 5e Monster Manual. As large as Big Red is, we find it unlikely even it is larger than the tarrasque. However, as mashtar says, “if the Red Dragon is considerably larger than the others then my bets on are Colossal.”