Summon Guardian Spirit: Size Increase from HD?

[Verecoth]

So I'm about to take the Summon Guardian Spirit in a game and I'm taking an Imp who will be gaining enough HD to reach more than double his original HD.

The Bestiary says that as a general rule any creature whose HD increases by 50% gains an increase in size. Would my Imp increase in size as he gains more and more HD from being summoned by a more and more powerful Summon Monster spell? Theoretically if this rule is followed he could end up being larger than my character, and I for one would really like this.

[Firebug]

I would lean toward no, as the template says what it gains from increasing its hit dice. Specifically: "It gains appropriate skill points, feats, ability score increases, base attack bonus, and base saving throw advancements for its increased Hit Dice." with no mention of size.
However, I would say GM call since the general rules actually leave that in the GMs hands. "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."

[Melkiador]

I agree with Firebug, but I’d also like to mention that increasing the size wouldn’t get you much of an improvement on the summon. So a GM shouldn’t worry about this overpowering anything. The only strength based familiar I can think of is the earth elemental. I might run the numbers later to see how size increases would effect it.

It’s also not clear if you can get multiple size increases from hit dice.

Quote:

As a general rule, creatures whose Hit Dice increase by 50% or more should also increase in size

200% more hit dice is still part of “or more”, so it would still fall inside that first size increase.

[Cevah]

Not sure why you would want to stay with a low HD base creature when you can change it out to a high HD base creature that still gets those bonus HD.

Here are some beefier ones:
Consular Imp: max 82 hp -- same as Imp but has telepathy and can change form
Hellfire Ignis: max 93 hp
Tidepool Dragon: max 120 hp
Earth Wysp: max 127 hp

That said, I would be OK letting you have the size, but without any stat change due to size. This would effectively only change attack range.

/cevah

[blahpers]

Monster Advancement -> Adding Racial Hit Dice wrote:

Adding racial Hit Dice to a monster is a similar process to building a monster from scratch. As additional Hit Dice are added, other abilities increase in power as well. Additional Hit Dice usually results in better attack bonuses, saves, hit points, and skills, as well as more feats. It can also include additional spellcasting capability and other powers.

Step 1: Plan the Monster

When advancing a monster by adding racial HD, you should start by deciding what you want the monster to become. In most cases, this means merely a tougher, stronger version of an existing monster. Note the desired CR of the new monster. This is also the point at which you should decide whether the creature is going to increase in size. 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.

The context of this "general rule" for increasing the size of a monster due to additional hit dice is for GMs designing new monsters by advancing existing monsters, not as a side effect of monsters gaining additional hit dice through other means (such as a template). If the template was intended to provide an additional size modifier, it would say so. Do not confuse GM design guidelines with player mechanics; that way lies madness.