A vertical spread's width — the distance between the long and short strikes — is its defining parameter. It sets what the spread costs, what it can pay, and how much it behaves like a plain single option. On a fast 0DTE trade, getting it right is most of the decision.
The trade-off width controls
A narrow spread ($1–2 wide on SPY) is cheap, decays slowly, and has a modest, quickly-capped payoff — a defined, low-cost bet to a nearby level. A wide spread costs more, pays more, and behaves increasingly like a single long option (more theta, more premium at risk), because the far short strike does less to finance the trade. Width is the slider between “cheap and capped” and “expensive and open.”
Match width to the target
The clean rule: set the short strike at your target level. If you're playing a grind up to the call wall two dollars away, a spread whose short strike sits at that wall captures the whole expected move and caps you exactly where you'd have taken profit anyway — so the cap costs you nothing real. Width beyond your realistic target just adds cost for payoff you won't reach; width short of it caps you before the move is done.
Don't pick a width — pick a target, and let the target set the width. The short strike belongs at the level you're aiming for.
Frame it against the move
Anchor the whole thing to the expected move. A spread wider than the day's realistic range is paying for territory the session won't cover; one that fits inside the expected move to a real level is sized to what can actually happen. And if you find yourself wanting a wide spread for open-ended upside, that's a sign a single option was the right tool all along.