That depends on your resolution. The common display resolution is 72 pixels per inch, or dots per inch (dpi). Sometimes it’s 96 dpi. Printing commonly requires 300 dpi.
Many web pages have larger images that are scaled down by the browser, you just set the image size on the web page. Just remember though, it will still have to be downloaded in its original state, so if it’s huge, it’ll take time to send.