I haven't done any modding to test it, but my understanding is that there's a minimum weight requirement for each weapon type...or actually two: below a certain weight, a unit can't equip a weapon. Then there's a weight range for equipping the weapon as a multigrasp (two-handed) weapon. Units above that weight range can equip the weapon in a single hand.
A spear in the default raws has the range:
[TWO_HANDED:47500]
[MINIMUM_SIZE:5000] (minimum is unusually low because spears are used by small animal people)
A pike has the range:
[TWO_HANDED:77500]
[MINIMUM_SIZE:62500]
A dwarf's growth is defined by:
(format is age followed by size, years:days:average size)
[BODY_SIZE:0:0:3000]
[BODY_SIZE:1:168:15000]
[BODY_SIZE:12:0:60000]
...So dwarves age 12+ weigh on average 60,000 urists, less than the minimum to equip a pike two-handed.
This is modified by the dwarf's height and "broadness", a randomly chosen percentage:
[BODY_APPEARANCE_MODIFIER:HEIGHT:75:95:98:100:102:105:125]
[APP_MOD_IMPORTANCE:500]
[BODY_APPEARANCE_MODIFIER:BROADNESS:75:95:98:100:102:105:125]
[APP_MOD_IMPORTANCE:500]
So the maximum is 125% weight due to being tall, 125% weight due to being broad, meaning a dwarf can reach 1.25*1.25*60,000 = 93,750 urists (assuming I'm right that they multiply with respect to each other).
Getting both 105% measurements would only give 66,150. Getting 125% for one and 102% for the other would yield only 75,000 urists, not quite enough. 105% and 125% together would be enough, at 78,750. So the two largest possible sizes for a dwarf would both allow single-hand wielding of a pike.
If you're going for it, look through physical descriptions for a large dwarf, assign them a pike, if it doesn't say "multigrasp" when they equip it, also give them a shield. Might be dangerous going shieldless.