The water effect works in texture space, it’s limited by the render target resolution. A 1000x1000 render target texture on a 1000x1000 unit mesh will give you 1 pixel per unit in the world. If scaling the mesh up to lets say 5000x5000 will result in 1 pixel per 5 units in the world. This is why you lose detail. The force splat draws in texture space also, this i why it scales up when you scale the mesh up.
For a larger effect you would need to increase the render target texture resolution and increase the plane mesh density.