Quote from a529612:
How about those trades that are marginally profitable? I mean those when a down day will send you back to the red.
Some of my big winners might show a small unrealized loss soon after purchase.
Say I buy a stock at $ 30 / share. My stop loss might be at $ 20 / share. I have a risk budget, say $ 100. I calculate position size so that if price decreases to $ 20 / share (30 - 20 = $ 10 / share) then I only lose $ 100. $ 100 / $ 10 / share = 10 shares. My position size is 10 shares.
I am comfortable because my risk is about $ 100, an amount that I am willing to lose.
If I bet much more, say $ 10,000 then I get scared and worry about small price changes.
Bet size influences the psychology of my trading.