The US Letter size (2550×3300px)
US Letter is the default paper size across North America at 8.5 by 11 inches, which at 300 DPI equals 2550 by 3300 pixels. It is what resumes, letters, contracts, and flyers are printed on in the United States and Canada, so if your audience is there, designing to 2550x3300 ensures your document fills the page correctly rather than being awkwardly scaled from A4, which is slightly narrower and taller.
That difference from A4 trips people up constantly. A file laid out for A4 (2480x3508) will not sit perfectly on Letter, leaving mismatched margins or forcing a shrink-to-fit that misaligns headers and footers. Sizing directly to 2550x3300 avoids that, and at 300 dots per inch your text, tables, and any logos or photos reproduce crisply on both office laser printers and professional presses. The 8.5x11 footprint also fits standard binders, folders, and envelopes sold in the region.
For anything with color or imagery to the edge, add bleed by extending the design a few millimeters past the 8.5x11 trim, and keep essential text inside a safe margin, since most office printers reserve a small unprintable border they cannot ink. For plain documents that stay within margins, the default 2550x3300 is enough. Everything here resizes 100% in your browser with no upload and for free, so your files stay on your device. Print shops vary in their bleed and color requirements, so confirm before a professional run.