The A3 Portrait size (3508×4961px)
A3 measures 297 by 420 millimeters, exactly twice the area of A4, and at 300 DPI that is 3508 by 4961 pixels in portrait. It is the standard step up for posters, larger flyers, presentation charts, and art prints that need more visual impact than an A4 sheet can give while still fitting many office and prosumer printers. Because two A4 pages tile perfectly into one A3, layouts move between the sizes without redrawing.
At this larger physical size, resolution discipline pays off. A poster is viewed from a distance but also up close on a wall, and holding the full 3508x4961 pixels at 300 dots per inch keeps headlines bold and photographs sharp rather than revealing soft edges when someone leans in. Upscaling a smaller image to fill A3 is the common trap: it looks fine on screen and then prints noticeably blurry, so start from a source large enough to reach these dimensions honestly.
Bleed becomes even more important at poster scale because any white edge is very visible. Extend background color or imagery a few millimeters past the 297x420mm trim, and keep essential text and logos inside a generous safe margin so nothing is lost at the cut. If you are printing at a professional shop, ask about their preferred bleed and whether they want crop marks. Everything here resizes 100% in your browser with no upload and for free, keeping your artwork private. Specs vary by printer, so confirm bleed, DPI, and color before ordering a run.