There's no single right size — it depends where the image is going
"What size should this image be" doesn't have one answer, because a photo destined for an Instagram post, a website hero banner, an email attachment, and a printed flyer each have different constraints driving the right choice — display size, file size limits, and viewing distance all pull in different directions. Sizing an image once and reusing it everywhere is usually the reason a picture looks great on a website but grainy when printed, or loads painfully slowly on a page that only needed a thumbnail.
Resize an image now
Social media dimensions
Most platforms recompress whatever you upload to fit their own delivery pipeline, but starting from roughly the right dimensions avoids unnecessary softening from an aggressive recompression pass. A square post (1080×1080px) is the safe universal default for Instagram and most feed-style platforms; a landscape 1200×630px is the standard for link previews shared on Facebook, LinkedIn, and X; a vertical 1080×1920px fits Stories and Reels-style full-screen formats. These numbers shift periodically as platforms update their own specs, so treat them as solid starting points rather than permanent, unchanging rules.
Web pages and page load speed
On a website, image size is a genuine performance issue, not just a visual one — an oversized image is one of the most common causes of a slow-loading page, since the browser has to download every pixel even if it's only going to display the image at a fraction of its native size. The practical rule: size an image close to its actual maximum display dimensions on the page (a 400px-wide thumbnail doesn't need a 4000px-wide source file behind it), and lean on compression on top of that, since resizing and compressing solve two different parts of the same file-size problem.
Resize and compress together
Email attachments
Most email providers cap total attachment size somewhere between 20–25MB, and a handful of full-resolution phone photos can approach that limit surprisingly fast — a modern phone camera routinely produces individual images in the 3–8MB range. Beyond just fitting under the limit, a recipient reading email on a phone gains nothing from a 4000px-wide image squeezed into a message viewport a few hundred pixels wide; resizing down to roughly 1200–1600px on the long edge keeps images sharp on any screen they'll actually be viewed on while keeping the whole message light enough to send and receive quickly.
Print vs. screen resolution
Screens and printers measure sharpness differently, which is the real source of most "why is my printed photo blurry" surprises. A screen displays at roughly 72–96 pixels per inch (PPI) at typical viewing distance, which is why a relatively modest pixel count still looks crisp on a monitor. Print needs considerably more density — 300 dots per inch (DPI) is the standard target for anything meant to be held and viewed up close, like a flyer or photo print — meaning a printed 8×10 photo at that quality needs roughly 2400×3000 pixels, far more than the same image would ever need for on-screen use. An image sized correctly for the web is very often too low-resolution to print cleanly at any real size.
The practical rule of thumb
Size for the largest way the image will actually be displayed, not for "just in case" — an image doesn't need to be any bigger than its biggest real-world use, and keeping a higher-resolution original around separately means it's easy to resize down again later for a smaller use without losing anything. Resize down for web and email, keep resolution higher for anything headed to print, and check a platform's current specs before uploading anything meant to appear a specific way, since those numbers do shift over time.