I've got a river that twists and turns and I was wondering if it is better to make one giant water brush that gives me all the water surface area I need (with a lot of extra water due to the size of the brush), or is it better to make the water out of 2-3 smaller brushes that match the path of the river better?

Make it so it matches the bed of your river, or you'll have un necessary water beneath the river banks, which really isn't needed.

Make it from seperate brushes, otherwise when the engine is rendering the cubemaps and so on it will have to do the pieces 'under' the map aswell which is wasting resources.