What decides how big water drops can become?
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What do you define as a water drop? Is the ocean not just a giant water droplet?
The problem is that, in conditions where water drops naturally form (clouds, slowly/intermittently running water, condensation), there are a balance of forces at play that keep water drops from getting too big.
Water drops in clouds that grow too big become too heavy and fall from the cloud; on the way down, they can also be fractured by air resistance into even smaller drops.
Water drops from intermittently running water (like on leaf/branch or coming out of a leaky faucet) have to fight gravity; the bigger the drop, the heavier it is, and the harder it is for the water’s adhesion to the surface to hold it up.
Condensate like dew is a bit of both of the above. Water can continue to condense on a blade of grass until it’s too heavy for the grass to hold it, at which point the grass bends and the water drop falls off.
The issue with a water drop is not the size, but the shape. It must be spherical, and ultimately at a certain size water doesn’t bond as strong compared to the smaller size.
Drops are held together by surface tension and pulled apart by air resistance. Also, the square-cube law operates to limit drop size. If you double the size of a drop then its surface area and wind resistance will quadruple (square law) but the volume and weight of the drop will go up by a factor of eight (cube law). So bigger drops will try to fall faster and get broken up by the air into smaller drops.
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