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Combustion chamber geometry matters more at low speed, like people said you need a big stroke and relatively small bore. If you increase per cylinder displacement, I imagine the stroke (and thus crank, rods, and overall engine height) can get out of hand and your engine will be physically too large.
To slow down flame speed you can use cooled EGR on a spark ignition engine, and ceramic coatings for the heads and pistons can reduce the amount of heat that is rejected to the cooling system, but I wouldn't be surprised if these aren't enough to get your typical light vehicle engine to work happily below 1000rpm.
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