How Many Added Hit Dice Warrant a Size Change?

[Randarak]

The title speaks for itself. I can find reference to it in monster advancement, but is there a standard that I am missing?

In other words, and for example, how many hit dice would I add to a mimic (which starts at 7d8 and Medium size) before said mimic advances to being of Large size?

Would I add the same number of hit dice again to advance it from Large size to Huge size?

Is this left entirely to GM discretion?

[GM Rednal]

Quote:

As a general rule, creatures whose Hit Dice increase by 50% or more should also increase in size, but GMs should feel free to ignore this rule if warranted by the individual creature or situation.

[Randarak]

Thank you. Sometimes I just need a mental nudge...

[GM Rednal]

It's in the Monster Advancement section, under Adding Racial Hit Dice.