Colossal is whatever I decided it is! There is no colossal in 5e, but for the purpose of my work it starts at 25x25 as noted in the Epic Damage by Ability Score table. Basically, colossal is anything over gargantuan (20x20) which may need special rules because of size. That is why it is a trait and specifically not a size. Notice everything with the colossal trait is still Gargantuan in size.Isn't 30x30 small for a colossal creature? I thought the minimum was 60x60.