I wasn't sure if this should go in Homebrew or Min/Max, but since the following charts, tables and conclusion are key to a number of different optimizations (monk builds in particular, but anything manipulating size), I figured this was the better place for it. Mods, please move this topic if I chose incorrectly!
Anyone who's tried to optimize a monk ends up
frustrated once they reach 20th level equivalent fists. Errata and sources outside PHB1 say that monk fist damage caps at 20th level equivalent, and the game changes to "how fat can I make my fists?" as a means to increasing damage. Unfortunately, DMG1 does not explicitly state that there is a maximum to how big weapons can get, only an effecitve minimum: once it's too small, it doesn't deal any damage at all and is not considered a weapon.
So what happens when we reach Colossal+ size? The rules aren't very clear. When we get a damage die increase from a class feature, is that an increase to the size category or to the "weapon category"? Sometimes, increasing a weapon's damage directly is not the same as increasing the weapon or wielder's size. Sometimes, it gets downright ugly. Sometimes, we use dice other than d6s and d8s. I will attempt to explain that d10s are what break fluent conversion between "weapon category" and "size category" for damage bonuses, and all other unusual die sizes are fairly easily explained by attempting to use the most efficient number of dice possible.
PHB1 provides a pair of very important tables (combined and reproduced below) on page 28:
Everything looks fine up until we get to medium weapons that use two dice for their standard damage, then things start to get a bit messy. The 2d4 and 2d6 weapons are functionally equivalent to their 1d8 and 1d12 counterparts, EXCEPT for the fact that they use two dice. For any two medium weapons with equivalent maximum damages, their damage scales equivalently by size. If we strike either pair of rows 1d8 & 1d12 or 2d4 & 2d6 we get a better look at what's happening: Typically, when there is an increase to the weapon category or size category, damage is equal to double the category two steps lower. I've heard this echoed in various forums when building super-colossal monsters and characters, and it seems to be the norm.
But how do we get from alternating d6s and d8s to just d6s or just d8s when we reach higher damage categories? This is because D&D rules appear to attempt to use the most efficient number of dice to represent the maximum possible damage. This means using fewer dice of a larger size as opposed to more dice of smaller size. Using more, smaller dice gives the player an advantage when dealing damage, since it increases the minimum (and average) damage very quickly. It "narrows the spread" so to speak. If we take the dice rolls listed in the table and convert it into maximum damages, everything becomes much, much clearer:
Everything scales very nicely according to the "twice the damage as two steps back" concept except when a D10 gets involved. D10s are only used in 5 of the spaces on the chart and throws off the scaling. The chart has to compensate somewhere, and it does so by making the huge and colossal damages for d10 and d12 weapons functionally equivalent. I further believe that the d10s are the exception, and not the standard, because they disrupt the flow of the chart elsewhere. If we use the "twice the damage as two steps back" method to rewrite the chart, and assume that all of the dice up to 1d8 are correct, we end up with the following:
This chart follows the formula 2^((N/2)-1)d(N%2+2), where N is the damage category of the weapon used and rounding down on division. For those unfamiliar with the modulus operator, it's like division, only the remainder is your answer. This will output damage in terms of d2s and d3s, which can be substituted for with larger dice as appropriate. Remember, this seems to be what the rules originally intended to do. What was once a 12d8 colossal level 20 monk fist is now a 16d8 colossal level 20 monk fist. A 1d10 weapon is now 1d12. Most standard weapons remain the same, and we can rewrite the chart as follows:
When trying to fit unusual weapons, such as the 1d10 polearms or 2d10 monkfists, my suggestion is to determine the weapon's maximum random damage and choose the nearest option on the list, rounding up on ties. A 1d10 (10) becomes 1d12 (12). A 2d10 (20) becomes 2d12 (24). A 3d6 (18) becomes 2d8 (16) NOT 4d6 (24).
When we get into Colossal+N sizes, damage spikes exponentially, as it likely should, and there is now an easier way for determining how absolutely squished your gish is (or isn't) when the enlarged, expanded terrasque stomps on you. Or, if you're even gutsier, how much more punch is in your fist when the level 20 monk slaps on monk belt, monk tattoo, and gloves of the talon, then starts taking epic levels in Fist of the Forest.