The original image size (i.e. 'full') is never modified. Intermediate image sizes are controlled in various places:

1. Default image sizes ( 'thumbnail', 'medium', 'large' ): core settings, via Dashboard -> Settings -> Media
2. Custom image sizes: Theme functions.php file, via add_image_size()
3. Post Thumbnail size ( 'thumbnail' ): Theme functions.php file, via set_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.

The original image size (i.e. 'full') is never modified. Intermediate image sizes are controlled in various places:

1. Default image sizes ( 'thumbnail', 'medium', 'large' ): core settings, via Dashboard -> Settings -> Media
2. Custom image sizes: Theme functions.php file, via add_image_size()
3. Post Thumbnail size ( 'thumbnail' ): Theme functions.php file, via set_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.

Edit

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:

• Resize At Upload
• Resize At Upload Plus
• Resize Images Before Upload
• And many more...