I made a hump in my pond that comes 12"-18" from the surface. It is not big, maybe 10X15, and I planted two cypress trees on top of it. And it does have grasses growing on it. This morning I cut down a forked tree that had laid over in a storm. Since it was right there, I cut two good sized logs and pushed them out into the pond where they are laying across the top of the hump. All seems to be working well.
As far as size, etc, I would say more smaller ones are better than fewer large ones, but that is definitely not a scientific opinion.
