For multiracial forts, are there any issues regarding default armor sizes and non-dwarven troops? Also, when you change a unit's race, say from dwarf to human, what happens to their clothing? Does it change, too, or are dwarves and humans close enough in size to share clothes?
The size of the clothing is determined by the size of its creator. Thus, dwarven smiths (and tailors) produce items appropriate for 60kg creatures, while a human would produce 70kg-baseline items. Clothing/armour cannot be shared between creatures of different sizes, although weapons can be (subject to 2-handed considerations, of course).
I haven't tried to alter a creature's race on-the-fly, but I have altered the bodysize of a caste. In my case, the creature retained its previous clothing but this state was precarious: when the soldier went off-duty and doffed his armour he was subsequently unable to re-equip it due to the size mismatch.
Depending on your circumstances, you might want to deliberately train up crafters of a particular race (e.g. exclusively human armorers), or train up a diverse group in order to produce a variety of equipment for a multiracial army. Alternatively, you could alter the raws in order to give adult humans and dwarves a common bodysize, but doing so could render much of your current equipment unusable (depending on which creature's definition you choose to modify).
I don't think that there's any way to ascertain the {bodysize} value for a piece of clothing that's simply lying around, but by using the dfhack "Set Current Race" function you can assess its size relative to various races. If you've set your race to "HUMAN" and the item name shows neither "small" nor "large", then it's the correct size for a human.
An interesting side-note: the cube-squared law (or something akin to it) takes effect when you apply this idea to non-standard creatures. For instance, a bronze breastplate made by/for a horse masses ~160kg, but the horse's strength doesn't scale up - the raw files "expect" non-humanoids to run around naked, and the numbers are balanced accordingly. Ironclad elephants are possible, but they move very slowly due to the immense weight of their armour. If the effect becomes game-breaking then you can compensate for it by boosting the creature's (or race/caste's) stats. Reducing the density of the metal is another option.