Position sizing: why one card shouldn't decide your month

August 13, 2026

Ask ten collectors which card they regret, and most will describe a good pick that was simply too large a share of their portfolio. The card being right or wrong mattered less than how much was riding on it.

The 40% problem

If a single card is 40% of your tracked portfolio, its ordinary volatility becomes your volatility. A card can be a completely reasonable hold and still be an unreasonable concentration.

Compare that to the same card at 6% of a portfolio: the position can still move sharply without threatening the month. Same card, same market, radically different risk — because sizing, not selection, was the variable that changed.

Sizing is a decision, not a byproduct

It's tempting to let size be whatever conviction and available cash happen to produce. But conviction is not a risk unit, and available cash is not a plan. Sizing deserves its own explicit decision, separate from "do I like this card":

• What's the largest single position you'd hold if you were wrong and didn't find out for a month?
• Would a 30% drop in this one card change your answer to the question above?

If either answer makes you uncomfortable, the position is oversized — regardless of how good the thesis is.

Trimming isn't losing conviction

Reducing an oversized winner back toward your target size isn't a bet against the card. It's separating "I still believe in this" from "I'm still comfortable with this much exposure" — two different questions that concentration collapses into one.

The takeaway

The market doesn't punish being wrong about a card nearly as often as it punishes being right about a card that was too large a share of everything else.