How do you determine the growth of something going from "medium" to "large"?

[eyebreaker7]

"A giant bombardier beetle is about 6 feet long."
"Advancement: 3-4 HD (Medium); 5-6 HD (Large)"

Going by the size adjustment chart it says medium is 4-8 ft. and 60-500 lbs.
Large says 8-16 ft. and 500-4000 lbs.

I'm guessing this is what you go by? Or is there something else I should be looking at?
Does HD range mean anything? Like HD 5-6 (Large), does it mean that the creature is 8-16 ft. or is it 8-12 ft for HD 5 and 13-16 ft. for 6 HD?

I'm looking for something a little "different" for a dwarven wizard mount. Not sure if I want an insect or an animal. An animal can be dire so there's that.
How do you determine how much a different sort of mount costs? I'm thinking maybe a giant bombardier beetle, or maybe a lion? Dire lion would be too big. Also I'm thinking maybe celestial? Again, how do you determine the additional cost for it being celestial?

This scarab beetle got me thinking about one.
https://village.photos/images/user/c305cbbe-e053-4c83-8f55-e8d66ccfa88f/d19b9caf-918f-445a-877f-3ff562ea3262.jpg

[Biggus]

The height/length listed in the size category table is more of a guideline than a hard-and-fast rule. Some creatures such as purple worms and black puddings aren't the size category you'd get from looking at their listed length.

Also, a creature's maximum HD doesn't necessarily mean that they've got to the maximum extreme of that size category.

Taking the specific example of the giant bombardier bettle, it's shape isn't particularly unusual so it probably is about 8ft long when it advances to large. As it already starts in the middle of the medium size category, that suggests it gains 1ft in length for its first two HD. As it gains only a further two HD as a large creature, that suggests it gets no further than the middle of the large category, ie 12ft. Or, it might just gain a further 1ft per HD. So it's length at 6HD is probably somewhere between 10ft and 12ft.

But even this is giving the size category system more credit than it's due; there is absolutely no consistency to how much a single HD makes a creature grow. Some creatures can be advanced by as little as 4HD to go from the bottom of a size category to the bottom of the next, others can advance 30HD or more without gaining one.

So the nearest I can can give you to an answer is "probably 10-12ft, but could be anything really".

[hamishspence]

Black puddings are the right size based on weight though. So are purple worms.

The general size guidelines are for quadrupeds and bipeds - and for quadrupeds it's nose to base of tail, not nose to tip of tail.

The black pudding and the purple worm, are neither bipeds nor quadrupeds - so other considerations come into play. The black pudding fits neatly onto a 15 ft Huge base, so it works. The purple worm is a bit oversized for a 20 ft Gargantuan base - but that's more an issue with the fact that bases don't scale up as quickly as creature lengths do - the worm ends up being treated as partially curled up.

[Remuko]

Black puddings are the right size based on weight though. So are purple worms.

The general size guidelines are for quadrupeds and bipeds - and for quadrupeds it's nose to base of tail, not nose to tip of tail.

The black pudding and the purple worm, are neither bipeds nor quadrupeds - so other considerations come into play. The black pudding fits neatly onto a 15 ft Huge base, so it works. The purple worm is a bit oversized for a 20 ft Gargantuan base - but that's more an issue with the fact that bases don't scale up as quickly as creature lengths do - the worm ends up being treated as partially curled up.

makes me miss 3.0 when sizes werent all forced into squares. i remember when horses took up 2 squares not 4.

[ciopo]

horses are 2x2 ? :O

[Remuko]

horses are 2x2 ? :O

yup in 3.5 all things are squares. 1 square, or 2x2 or 3x3 or 4x4 or 5x5 etc. I miss when a 100 foot long worm that was 10 feet across was actually 2x20. now its like 4x4.