Yes, that is all they do, talk about it. The reason why some choose the ES is because it offers good trading opportunities, meaning lower risk for higher reward. It is like everything else, most will try and fail, a few will try and succeed. Those who succeed do so because of what they know, those who fail do so because of what they don't know. Simple really!
The ES can accommodate size, if such accommodation is not required there are other markets offering more bang for the buck.
Yes, number of contracts. The number would be such as one would be uncomfortable with slippage in a market with less contract volume.
I don't trade a very large number so I don't know where slippage would become material. I will simply ramp up until such a point it does. I will say that when I traded the old ER (Russell 2000), I would get uncomfortable above 6 or so but that was a much thinner and faster market.
So you would not be comfortable trading GC or CL or NQ with 6 contracts because you are afraid of getting slippage, even when their daily volume is 200-500K?