tl;dr
Make dwarves size 70000 and use 69:88:91:93:95:98:116 for their height and broadness modifiers instead of 75:95:98:100:102:105:125. They'll still be dwarf-sized, but be able to use human weapons, and armour.
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Why This Matters----------
Currently, the ability to wield a weapon or not is based on a creature's racial size and not its individual size. Making dwarves size 70000 allows all of them to wield any weapon, like a human, but also makes them human-sized. Manipulating the height and broadness of a dwarf allows one to reduce these oversized dwarves back to their "proper" size. For the sake of simplicity, it will be assumed for the calculations below that the values for height and broadness are identical, ie. a dwarf of average height will also have average broadness. In the game itself, a dwarf may be of above-average height and below-average broadness, and so on.
Height and broadness of a dwarf may be up to 25% higher or lower than the racial average, with most dwarves being around this racial average. The game chooses these numbers by randomly picking a size category, then randomly picking a number within this category. Shown below are these categories, with the size of a dwarf on the edge of these categories also shown.
Racial size: 60000
Category: 75 95 98 100 102 105 125
Size: 33750 54150 57624 60000 62424 66150 93750
The middle figure does not have to be 100. This allows us to change racial size without having to change the size of an average individual of that race.
Racial size: 70000
Category: 69 88 91 93 95 98 116
Size: 33327 54208 57967 60543 63175 67228 94912
The average size is changed somewhat, but this is unavoidable. Each of these categories was chosen for producing the size closest to the unmodded dwarf, with some being larger and some being smaller.
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Arena Testing----------
Because one can never assume that things will work as written in DF, testing was done. 100 normal-sized dwarves were temporarily made [AQUATIC] for the production of undamaged corpses, and butchered after air-drowning. Then, the same was done with 100 altered-sized dwarves.
All 200 dwarves produced the following:
16 fat
Skin
Skull
Brain
Nervous tissue
2 kidneys
Spleen
Pancreas
Stomach
Liver
Intestines
Heart
2 lungs
Only the amounts of bone and meat differed. In each test, there were only four combinations of butchery returns, as named below.
Racial size: 60000 | Racial size: 70000
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Bone Meat | Bone Meat
|
Small: 19 20 | 19 20
Average: 23 20 | 23 20
Large: 23 22 | 23 22
V. Large: 33 30 | 33 30
These were identical between tests. The frequency of each combination did vary, as shown below.
Racial size: 60000 | Racial size: 70000
|
|
Small: 11% | 4%
Average: 70% | 69%
Large: 11% | 20%
V. Large: 8% | 7%
These differences might or might not be significant due to the way butchery works (in short, individual parts of a creature produce one unit when that body part is large enough, and increasing the size of a creature might or might not produce a second unit, and might or might not produce a first unit from a formerly too-small body part). Increasing dwarven size tenfold would produce more fine-grained information, but create more combinations of butchery returns.
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In Closing----------
Honestly, I'm pretty happy with the results. The dwarves remain close in size to the originals, and while the average size is bumped up a bit the extreme areas where the 1H / 2H weapon wielding limits are are less affected. If you did two forts in the same world with these two different sizes, the size differences between the inhabitants would probably be less than the variation found between civs.
For example,
these dwarves were small, and the ones in my current fort are large. Out of 30, 11 are below the dwarven average and 7 are above the human average. Nine have a Strength score below 1000, so the large average size of the fortress isn't down to muscle mass.
Now I'm going to enjoy that *iron great axe* the humans brought.