I would say if you have enough of them leave them as big as possible so they will last longer. You told how thick and how wide but did not say how long they are. If they are long enough and have enough of them you can also use a slab driver and drive them in the ground of the lake to make a stake type bed also. Picture of slab driver and here is the youtube:
https://www.youtube.com/watch?v=ScovIBC64t8
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