Powerful Build does not affect weapon damage
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 already tells you how it affects weapon damage
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.
Transient size changes don't affect weapon damage unless they say so
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.
A merrow's harpoon probably does 1d6 damage in a PC's hands
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).