The three standard sizes
Almost all direct-mail postcards come in one of three sizes: 4x6 inches, 6x9 inches, or 6x11 inches. Each one trades off three things — what it costs to mail, how much room you get for your message, and how much it stands out in the stack of mail.
4x6 — the everyday workhorse
The 4x6 is the smallest and cheapest of the three. It qualifies for USPS First-Class postcard rates, which is part of why it's so economical, and it's perfect when you have one clear message: a recall reminder, a single offer, a quick promotion.
Because it's the lowest-cost size, it's the right choice for campaigns you run often — monthly reminders, ongoing reactivations — where keeping the per-piece price down matters more than maximum presence. On CardCadence the 4x6 is $0.75 a card, all-in.
6x9 — more room, more presence
The 6x9 gives you noticeably more space for the offer and a bigger footprint in the mailbox, so it's harder to overlook. It's the natural step up when a single tight message isn't enough — you want a headline, an offer, and maybe a small image or a few proof points.
It mails at a higher rate than the 4x6, reflected in its $0.80 per-card price on CardCadence. For many businesses it's the sweet spot between cost and impact.
6x11 — the big statement
The 6x11 is the largest standard size and the hardest to ignore. It dominates the mailbox, which makes it well suited to moments that warrant it: a grand opening, a premium offer, a can't-miss seasonal push.
It's the most expensive of the three ($1.13 a card on CardCadence), so it's usually a deliberate choice for a high-stakes drop rather than your everyday reminder.
How to actually choose
A useful rule of thumb: match the size to the frequency and the stakes. Frequent, simple, cost-sensitive drops lean 4x6. Important or content-heavy drops lean 6x9. Big, infrequent, high-impact drops lean 6x11.
Better still, test. Mail the same offer to two comparable halves of a list at different sizes, put a trackable QR code on each, and compare the scans. That replaces opinion with your own data — and since every CardCadence card is trackable by default, the comparison is built in.