there's an element of luck involved in this game. I'll readily admit it. Both good luck and bad luck (like getting low ticked/stopped out and then have the market go your way just after.
Your example has absolutely nothing to do with luck. Your example has to do with inaccurate data analysis. An example of luck would be entering the market and a piece of news, which you have no control over, content, timing or otherwise, moves the market in a good or bad way in relation to your entry.
One of my favorite life slogans is "The more I do, the luckier I get". A close look at that statement, you'll see most so-called luck comes from experience and doing, although the uncontrollable is omnipresent.
if you continued trading at maximum size you're bound to have a bad day.
Unless you are talking about a method scalability issue or instrument liquidity constraint, maximum size has nothing to do with trade efficacy. You can sling 1, 5, 50, 100 contracts at ES for instance and it will have zero effect on how the trade performs.
The traders' mindset and discipline is the culprit in the scenario you speak, period.
That is all.
Carry On!