Can a PC use a Merrow's Harpoon? says that a Merrow’s harpoon’s damage changes based on the PC’s size. The answer mentions that because PCs are usually medium sized that the damage changes from 2d6 to 1d6.
So since it starts as 2d6 but the PC’s size makes it 1d6, what about if the PC has Powerful Build or is enlarged by enlarge/reduce?
The linked answer points out that the rules for players and for monsters are different and, thus, that there is no way of knowing what damage a harpoon does when a player wields it-- it has no stats for that. The GM could use the improvised weapon rules, but that's not really appropriate because it's not really a non-weapon object; it's a weapon for which you lack stats.
The linked answer then discusses what you might do if a player picks up and seeks to use a harpoon. That advice is informed by rules text, but it is not rules text or required by the rules. It's RAW compliant, because the rules say nothing about what a player-wielded Medium harpoon should do, but it's not the RAW.
Furthermore, the argument therein is for determining for the GM what damage the weapon should do when a player uses it. It's not that you're coming up with a new weapon property, "big", that makes a weapon do different damage for large creatures; you have a weapon with no stats and you are deciding "Ok, based off the Merrow's damage, this should probably do 1d6". It's the same as if the merrow picked up a Medium longsword and you wanted to figure out how much damage it should do with it. One of the options would be to look at the size damage guidelines in the DMG and decide "Okay, let's have the sword deal 2d4 as a light weapon with disadvantage and call it an awkward dagger".
This interaction, then, is limited to cases where your DM is using the other ruling the linked answer discusses, where you have a large-sized harpoon and you let Medium-sized players deal 2d6 with it but they have disadvantage on the attack roll. In this case, being Large (which Enlarge does accomplish but Powerful Build does not) has no affect on the damage the harpoon does, but would let you use it without disadvantage (provided you don't make the harpoon grow with you. If you do, then it does 2d6+1d4 and you still have disadvantage).
Powerful Build is a straightforward feature:
You count as one size larger when determining your carrying capacity and the weight you can push, drag, or lift.
The only effect of Powerful Build is to increase your carrying capacity (along with several related numbers that all depend on your carrying capacity). Unless you are attempting to carry, push, drag, or lift something, Powerful Build is irrelevant. Specifically, it has no effect on the damage of your weapon attacks.
Enlarge/Reduce clearly explains what effect it has on weapon damage. Specifically, when enlarging:
The target's weapons also grow to match its new size. While these weapons are enlarged, the target's attacks with them deal 1d4 extra damage.
No other changes to weapon damage are specified. If you hit with a weapon that uses a d6 damage die while enlarged, you roll a d6 as normal and then add a d4 to it, then add your damage modifier.
The answer you cite mentions that when determining the damage dice for a monster's weapon attacks, you add additional weapon damage dice based on how many steps up from medium the monster's size class is. However, not only does the rule apply only to monsters, it also only applies to determining the base damage of monster's attacks from its natural size. This rule does not apply to temporary size changes like those caused by Enlarge/Reduce. A monster enlarged by this spell would have its damage increased by a d4, just like a player character, and would not have any other changes to its damage. (Conversely, a monster shrunk by this spell would not lose a damage die, but would instead subtract a d4 from their damage rolls.)
We can also see that this from the duergar's Enlarge action, which explicitly says it doubles the duergar's damage dice for strength-based weapon attacks. That wouldn't be necessary if changing from medium to large automatically granted an additional damage die on the duergar's attacks.
The mathematics is adequately covered in the answer you have linked to, which concludes that the harpoon's damage is derived from a base damage of 1d6 for a medium sized creature. Neither Powerful Build nor Enlarge/Reduce changes this, although a medium creature being enlarged by the spell would be able to use the weapon without disadvantage (but only if they picked it up after being enlarged, so that the weapon does not also become enlarged).
