You can configure global resizing for uploaded images in Settings -> System -> Files -> Image resizing.
The system automatically reduces the size of images when they are uploaded according to the values that you fill in for the Automatic image resize on upload (width, height, max side size) settings:
The image dimension values are measured in pixels.
These settings are applied by default when uploading images as:
media library files
page attachments
rich text page fields– uploading images using the rich text editor dialogs.
Field editor fields – uploading images using the following field types (form controls):
The default resizing settings can be overridden by local settings in the particular parts of the user interface (e.g. field editor, editor dialogs, etc.).