Image size

This is an old ticket and I am closing it but leaving a response to the original question below:

Active Theme or Plugin – You cannot alter existing image sizes created by your theme or other third party plugins (as that resides in theme’s files or the third party plugin’s files).

WP Core – WordPress, by default, comes with a few image sizes (small, medium, large, and thumbnail) – You can edit these from within the WP Settings > Media settings.

New Image Sizes – You can create new image sizes with this plugin.

Thank you.