The Postcard size (1875×1275px)
A standard 6x4 inch postcard prints at 300 DPI, and 1875 by 1275 pixels gives you the finished 6x4 trim plus the small bleed area most postcard printers require. That extra margin is deliberate: postcards are cut from larger press sheets, so extending your background past the trim line prevents any thin white edge from appearing after the cards are guillotined, which is a common flaw on edge-to-edge designs.
Postcards are a direct-mail workhorse for promotions, save-the-dates, thank-you notes, and event announcements, and the 6x4 size qualifies for standard postcard mailing rates in many postal systems, keeping campaigns affordable. At 300 dots per inch, headlines, offers, and photography all print crisply, which matters because a postcard has seconds to grab attention in a stack of mail. The landscape orientation gives you a natural front for a bold image and a back for the message and address block.
Design with the postal layout in mind: the back typically needs a clear address area and stamp zone kept free of artwork, and both faces should keep essential text inside a safe margin from the trim so nothing is clipped. Because the front is often full-bleed imagery, this is exactly why the 1875x1275 canvas includes room past the 6x4 cut. Everything here resizes 100% in your browser with no upload and for free, so your design stays private on your device. Printers and postal rules differ, so confirm bleed, safe zones, and mailing specs before a bulk run.