Here's a pic of my pond when I did a total re-design a couple years ago - the flat boulder next to the electrical outlets is close to the size you're talking about - it's a trapezoid-shaped rock that's about 6" thick, 3' long on the long side, and 1' wide. The pond size itself is about 10' diameter irregular circle, with a 6' long stream and 3' at the deepest point. Approx 850 gallons after all the rockwork was put in place.

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This other pic shows the other side of the pond, with another somewhat large boulder in the top left of the pic. This was a max of 2' long, and probably 1' wide, but only about half is visible when the water is at the correct height (these pics were taken when I was still re-organizing and re-filling the pond).

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Hope that gives you some ideas.

Here are a couple pics of mine. I don't know of any formula to figure out size of rock verses water volume or size. If the rocks look to big you can always partly bury them.

My pond is about 20' x 25' with about 3000 gallons of water. Just finished building in 2008 to give you a sense of rocks verses water.
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After a couple years of plants filling out.
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Have fun and make sure and post a few pics.
Flora

Thanks to both of you for posting pics. Those are lovely ponds and that does help me visualize what it could look like - with alot of work ahead!