The rules are very unclear on this.
There are two questions we need to answer here:
- Do the various effective size increases you've listed stack with each other?
- If so, how do they stack? That is, how much damage does the resulting character actually end up dealing with their unarmed strike?
I'll tackle these one at a time, but fair warning going in: Both are a mess.
1. Do they stack?
There are two places in the rules that address stacking.
The core rules (appearing in both the PHB and the SRD) have the following to say about stacking:
In most cases, modifiers to a given check or roll stack (combine for a cumulative effect) if they come from different sources and have different types (or no type at all), but do not stack if they have the same type or come from the same source (such as the same spell cast twice in succession). If the modifiers to a particular roll do not stack, only the best bonus and worst penalty applies. Dodge bonuses and circumstance bonuses however, do stack with one another unless otherwise specified.
It's unclear whether these rules apply - they're about "modifiers to a given check or roll." Normally, this would mean something like "enhancement bonuses to attack rolls" or "circumstance penalties to move silently checks". However, a damage roll is clearly a "roll" of some kind, and increases or decreases to your unarmed damage based on size could be considered "modifiers" if you squint.
If these rules do apply, then the increases will stack. No type is ever given for these things (e.g., they're never described as a "size bonus" or anything like that). They just say something like (using Improved Natural Attack as an example):
The damage for this natural weapon increases by one step, as if the creature’s size had increased by one category
The second set of rules that might apply appears in the Expanded Psionics Handbook, p. 56:
Same Effect More than Once in Different Strengths:
In cases when two or more similar or identical effects are operating in the same area or on the same target, but at different strengths, only the best one applies. For example, a character under the influence of both the oak body power and the iron body spell benefits only from the stronger effect (in this case, iron body). If one power or spell is dispelled or its duration runs out, the other power or spell remains in effect (assuming its duration has not yet expired).
You might have some questions about this rule, like:
- Why does this rule appear in the section on magic/psionic transparency in the XPH? Does this mean it only applies to spells and psionic powers, or does it apply to everything?
- How similar do two effects have to be to trigger this rule? The given examples, Iron Body and Oak Body do some of the same things (natural armor bonus to AC, ability score modifiers), but also some different things (iron body makes you resistant to acid and makes you count as an Iron Golem for some purposes, while oak body instead makes you resistant to cold but vulnerable to fire).
The rules provide no clear guidance on these questions. If this rule applies, it would indicate that the size stacking you're describing wouldn't work, since "increase your unarmed damage as if you were one size category larger" and "increase your unarmed damage as if you were multiple size categories larger" are pretty clearly an example of "Same Effect More than Once in Different Strengths".
2. If so, how?
Once again, there are some relevant-looking rules, but they aren't really going to solve our problem for us.
Problem 1: The table for increasing damage via size doesn't go far enough.
The rules for increasing damage via size are described in a few places (e.g., the Improved Natural Attack feat), but the most authoritative version is probably Table: Larger and Smaller Weapon Damage (also appears in the DMG). This table lets you take your base damage if you were a medium-size character, and then tells you based on that what your damage will be at a given size other than medium.
This table has two issues for you:
- The base damage listings don't go high enough (they top out at 2d6, the base damage of a greatsword, but medium-size monks can have 2d8 or 2d10 base damage if they're high enough level)
- The size listings only go up to colossal. As you note in the question, you have enough size increases to take you several steps past colossal.
What are you supposed to do about this? The rules don't say. You can do some educated guessing via extrapolation from that table, but you're well into the realm of house rules at that point.
Problem 2: There's actually no such thing as "colossal++++"
The core rules only address size up to colossal. There are rules for going beyond this; they're on page 99 of Draconomicon and also appear on the SRD entry for Epic Dragons. But as far as I can tell:
- They're only written to apply to dragons
- They only describe very specific benefits (e.g., they don't say anything about how they affect unarmed damage)
- They don't address how going more than one step beyond colossal works (i.e., "colossal+" is a thing, but "colossal++" and beyond aren't)
So, when you're trying to calculate the damage of a Monk with 2d10 base unarmed damage who's four size categories beyond colossal...you've moved beyond the realm of what's covered by the rules.
So how would I rule?
At the end of the day, when the rules don't clearly cover something, I tend to fall back on "what would be good for the game?"
In D&D 3.5, unarmed combat melee characters, especially monks, are notoriously weak. And, in a game where spellcasters have tricks like "take an arbitrarily large number of actions per turn" and "remove multiple enemies from a combat, no save," allowing Punchy McPunchface to do a big pile of damage with his unarmed strikes isn't going to rock the boat.
So what I would do is:
- Allow the various effects you've listed to stack in a straightforward, additive manner.
- For base damage values that aren't listed on the weapon size table (e.g., those from monk's class-based advancement), increase die size by one for each step up. So, a monk who deals 2d8 base damage when Medium would deal 8d8 base damage when colossal, and a monk who deals 2d10 base damage when medium would deal 8d10 base damage when colossal.
- For size categories beyond colossal, extrapolate from the huge > gargantuan > colossal progression, and add two dice for each size increase.
So, combining all of the above, a level 20 Monk who counted as colossal++++ would deal 16d10 base damage. That's a lot! But it's also a lot of character resource investment, and it's no more than a Mailman style direct damage sorcerer, or a Power Attack-based charge build, or a high-op shadow pouncer, or any other character build that invests all of its build resources in doing a whole lot of damage.