Most sizing discussion stops at risk math — how many contracts keep your loss at your number. But there's a second, independent ceiling: liquidity. If you trade more contracts than a strike can comfortably absorb, your own order pushes the price against you, and no risk formula accounts for that.
Why size can exceed liquidity
On a tight-stop trade, the risk math can permit a lot of contracts (small per-contract risk → large count). But a thin or far strike may only have a handful of contracts on the bid and ask. Try to buy 30 contracts where 5 are offered and you walk the price up to fill — paying progressively worse, widening your effective spread. Worse, when you exit (especially a losing trade into a thin, widening market), you can't get out at a fair price because you're most of the volume. Your size becomes the problem.
The liquidity ceiling
Cap your size at what the strike can absorb without you moving it — a fraction of the resting size and typical volume. Favor liquid, near-the-money, round-number strikes where deep two-sided size means you're a small part of the flow. If your risk math calls for more contracts than the strike can hold, take the smaller number — liquidity wins. Being too big for a strike is a hidden cost that eats scalping edge.
Your risk math is one ceiling; the strike's liquidity is another. Take the lower of the two — being the biggest order in a thin strike is a tax you pay on every fill.
The practical rule
For most retail scalpers on liquid SPY strikes, liquidity isn't the binding constraint — but it becomes one as your account (and size) grows, or if you drift to illiquid strikes. Watch your fills: if you're consistently walking the price to get done, you're too big for the strike. NoVo routes to liquid strikes and sizes with execution quality in mind, because a fill you moved is edge you gave away.