Monster Advancement Question

[mikethepoor]

My friend and I are confused about what happens to a monster's size when it gains extra HD. I say the total number of HD determine whether its size increases or not; she says it's the number of HD added beyond the base total. According to RAW, are either of us right, and if so, who?

[The Necroswanso]

Size changes as listed:
15-30 HD(large)
31-40 HD(huge)
It means when their HD is within that set number they become that size. It's kind of like a level requirement, wink.

If a monster, (This is all gneralization) say a Red Dragon needed to be 35 HD above 20 to be collasal. That'd bee 55 HD, while alot, you'd never really be able to handle it barring a super epic campaign, and thus wouldn't have a collasal size listed anywhere except the epic Handbook really.

[kamikasei]

My friend and I are confused about what happens to a monster's size when it gains extra HD. I say the total number of HD determine whether its size increases or not; she says it's the number of HD added beyond the base total. According to RAW, are either of us right, and if so, who?

You're right, it's the total number of HD.

I don't think it's explicitly stated, but it can be seen by looking at any monster entry with an advancement by HD listed. For example, manticores have 6HD. Their advancement entry reads, "7-16 HD (Large); 17-18 HD (Huge)". Any other such monster's advancement entry will also start at one above their standard HD. That should be convincing, I hope.

[FlyMolo]

What kamikasei said.

It's easy to advance something by a single HD, to up the challenge for your players without getting scarier abilities. But having to double the HD for something just to get a size increase? Seems excessive.

So they're ranges from zero, not from however many HD it had before.