According to the calculations I made based on the raws, about 3 of 49 dwarves, on average, should theoretically be large enough to wield a halberd
in one hand. The number that can wield a halberd should be significantly larger, so I'm surprised to hear that you're having trouble finding even one dwarf that can wield a halberd two-handed.
The size mods for dwarves are 75%, 95%, 98%, 100%, 102%, 105%, and 125%, with an even chance of all seven, according to the comments in the raws. There are two sets of them, one for height, one for broadness. Obviously, the largest mod makes much more difference than the other two, but even 60,000 * 1.05 = 63,000, so a dwarf with average height, and second highest possible "broadness" (or vice-versa) should be able to wield a halberd two-handed.
More thoroughly, a dwarf that has one 125% mod can get away with all but the smallest mod, so that's (1/7) chance to get the 125% mod for strength, times (6/7) chance to get something high enough for broadness = 12% chance, same goes for broadness, but we have to subtract the (1/7)^2 chance of getting both (since we'd count it twice if we just multiplied by two), which gives us
((1/7) * (6/7) * 2) - ((1/7) * (1/7)) = 22.4% chance.
In addition, if the dwarf didn't roll 125%, but did roll 105% for one modifier and 100%, 102%, or 105% for the second, that's also enough to wield a halberd (we're already counting the 105% * 125% chance, so we don't include it). Once again, we double it because we could roll the 105% for either height or broadness, and subtract (1/7)^2 because we're counting it twice, that gives us ((1/7 * 3/7) * 2) - (1/7 * 1/7) = 10.2% more.
102% * 102% isn't enough, so that's the complete chance: about 32.7%.
So almost one in three dwarves (32.7%) should be able to wield a halberd in two hands.
I'm not sure whether it's easy to see who's large enough, there's some sort of lottery involved in deciding what personal appearance modifiers are worth mentioning in the description, and the size ones don't always win that lottery, so you may not be able to tell who's large enough until you order them to pick up weapons, since other personal appearance features could be more important to the dwarf's overall appearance, it might not be mentioned that they're large or small.
Now, if strength plays a role in weight, you might well need to strength train them first, because a lot of dwarves start out as weaklings, which would mean they'd likely be getting negative percentages from their low strength numbers, and you might need to get their strength up to average before they'd reach the ability to wield a large weapon.
Some have suggested that fatness may also play a role, but I have no idea how you influence fatness, or whether fatness in descriptions is one way that "broadness" makes it into physical descriptions. If dwarves start out lean, or become lean in the process of increasing their strength, and fatness is a factor in weight (which I haven't seen confirmed yet), then it might be more difficult to get them properly prepared to wield large weapons.