The original image size (i.e. 'full') is never modified. Intermediate image sizes are controlled in various places:
- Default image sizes (
'large' ): core settings, via Dashboard -> Settings -> Media
- Custom image sizes: Theme
functions.php file, via
- Post Thumbnail size (
'thumbnail' ): Theme
functions.php file, viaset_post_thumbnail_size()`
By default, the
'large' image size is constrained to 1024px. So, you can simply output this image size where you need a 1024px image.
If you need to create a custom image size, you'll just need to regenerate thumbnails for existing images.
If you need/want to constrain the size of the
'full' image size, then you'll need to modify it during the upload process, i.e. by hooking into
wp_handle_upload, and performing the resize.
There are various Plugins that accomplish this. A quick search turns up: