The liquidity in the Nasdaq is not as good as the S&P, but both are fine. Our size just under 100 contracts and we don't ever have more than 1 point slippage in the Nasdaq and about half a point in the ES. We tend to not trade at noon though, but if we did that number would probably be worse. In addition to the contracts we directly trade there are an additional 50 to 200(depends on the contract, risk, time of day etc) that go off from people in our chatroom, and again there tends to not be too much trouble with liquidity. The Mini's tend to jump around a lot, and if you have size putting in a limit order gives the market something to lean on and you can get filled pretty easily with them. We have a much harder time with Market Orders, which is not the case when with small sizes.
Brandon