Changing default image resize dimensions in Wordpress

How can I 开发者_StackOverflowmodify the core Wordpress files so that I can specify different default dimensions for the image resizing function in posts? For instance I'd like the 'Medium' option to have a default width of 400px instead of 300px.

Thanks

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Do you have to do it via core wordpress files? You can do it through the UI by going to Settings | Media | Image Sizes.