How do I scale up reach when designing a larger monster?

There is a creature I am designing for a 3.5e game, it has both weapon attacks and natural attacks. It also changes size as it gets more damaged.

The reach for the weapon attack follows the reach for tall creatures, as in the core rulebooks. What I am having trouble is scaling the natural attacks.

When it is medium size, it has

Attacks / Reach

• Iterative by Weapon (primary)

• 2x Tentacles (secondary) 1d6 reach 15ft

How would the tentacle reach scale up on each size above medium?

The Monster Manual says you're out of luck. On Space/Reach says that

A template may change this [space/reach] entry if it changes the monster’s size. The typical space and reach for monsters of each size is given on Table 7–1: Creature Sizes (page 314). Note that this table does not take into account special situations such as a roper’s strands. (292)

Then the Monster Manual takes this a step further in the section Creating a Monster on Space/Reach, saying

Some creatures have exceptional reach due to a particular weapon choice or a quirk of anatomy. It’s all right to assign a special reach figure that applies to a particular attack (such as the roper’s strands). (301)

(These sections are referenced because the section on Size Increases (291) fails to mention reach at all!)

Somewhat fortunately, there is an example of a creature that possesses tentacles that have greater reach than the rest of its natural attacks and that sees its size category increased: the Monster Manual section Example of Monster Advancement (292) shows an otyugh going from 6 Hit Dice to 15 therefore from its normal size category of Large to its new size category of Huge. The Large otyugh possesses a 10 ft. reach normally and tentacles that possess a 15 ft. reach, while the Huge otyugh possesses a 15 ft. reach normally and tentacles that possess a 25 ft. reach!

So the only thing readers really know is that game's designers thought that a Large otyugh's extra 5 ft. of tentacle reach should become an extra 10 ft. when the otyugh becomes Huge. But it's a mystery if that applies generally and a mystery if a (somehow) Gargantuan otyugh's 20 ft. reach (as per Table 8–4: Creature Size and Scale (Player's Handbook 149)) should be 30 ft. with its tentacles or should scale to 40 ft.

The Monster Manual says twice that it's just not going to supply this information. That's enough for this DM to make it up himself. I recommend you do the same.

• And then suddenly there is a colossal tentacle with 90ft reach whose tip moves so fast it deals sonic damage in a XXft area because it goes well over Mach 1 when it snaps. [Insert here remark about real physics in a DnD game] - oh, well, nice digging, senpai. Here's a green checkmark. Jan 29, 2018 at 23:29

In a related question about whips in Pathfinder, I wrote an answer saying

No one knows.

For regular reach weapons, your reach is doubled—so when you get bigger and your natural reach increases, your reach-weapon reach increases proportionally.

But whips are a weird exception: it just says it has a 15-foot reach. You could be overly literal and interpret that to mean that whips always have a 15-foot reach, no matter the size of the whip and wielder, but that’s remarkably inconsistent with every other weapon in the game, not to mention pure nonsense with respect to how whips work (then again, not being able to threaten with a whip is if anything even more nonsensical—“attacks of opportunity” are literally the only thing a bullwhip can actually be said to be good for in combat).

So no one thinks that the reach should stay at 15 feet. The question is, what does it become?

The first Google hit for this question is this Paizo thread, and literally the first two responses assert two completely different interpretations:

• The whip gives you +5 ft. more than you would normally have with a reach weapon. So if a Large creature normally threatens up to 20 ft. with a reach weapon, with a whip it threatens 25 ft.

• The whip gives you triple your natural reach. So if a Large creature normally threatens up 10 ft. without a reach weapon, it threatens 30 ft. with a whip (instead of 20 ft. with a regular reach weapon).

There is nothing in the rules that covers which of these two options is the case, and to my knowledge no Paizo developer has ever commented on the problem (James Jacobs has, but he is the creative director, not a rules manager—YMMV on his authority here). Ultimately, they may be ignoring the issue because whips are nearly unusable and so it probably shouldn’t ever come up.

The only part of this that doesn’t apply to your creature’s tentacles are Paizo’s statements, and my commentary that whips are nearly unusable.1 Unlike Pathfinder, where there is at least James Jacobs’s statement for whatever it’s worth, 3.5 lacks any explanation for greater-than-normal reach and how that scales with creature size.

In the end, you will have to make a judgment as DM to determine what provides the level of challenge to your PCs that you are going for.

1. It’s still true in 3.5, mind you, but a tentacle isn’t a whip, and lack the problems that whips have.
• A tentacle isn't a whip, the tentacle threatens, does not provoke AoO, and lack the problems the whip have. So the comparison is not valid, except for the fact that a tentacle is long and thin and so is a whip. But also so is a dragon's neck. Dec 26, 2017 at 19:46
• @Mindwin Literally everything asked about in this question and that one is identical between a whip and this tentacle: that it has a stated, explicit reach of 15-feet at Medium size, and no further information about how that should change as the creature size changes. I explicitly stated in the section after the quote that the bits about whips being unusable doesn’t apply to the whip, but that bit is entirely tangential to the quoted answer anyway. Dec 26, 2017 at 19:51
• A whip has no reach (it doesn't threaten). It is a badly designed adhoc weapon that can attack whithin 15 feet but feels more like a bad ranged weapon feat sink. Dec 26, 2017 at 19:57
• @Mindwin A whip most definitely does have reach; its inability to threaten is separate from its reach. It is defined as having a 15-foot reach for a Medium-sized creature. So is your tentacle. There are no rules for how the whip’s reach changes with creature size. This is also true of your tentacle. Hence, this answer. Everything else about whips, including its lack of threatening, its poor design, its uselessness, and so on, is irrelevant to this question and thus to this answer. I have explained this in this in my answer. (Also, 3.5 doesn’t have feats for whips; you might be thinking of PF.) Dec 26, 2017 at 20:25
• (And as for whips feeling like a weird ranged weapon, in 3.0 that’s exactly what it was, oddly enough. I disagree with your assessment of the 3.5/PF whip on that front, though.) Dec 26, 2017 at 20:26