Changing the default image size when adding media

When I upload media into a page, the default size is 300 X 255.

Is there a way to make the default size 100%?

Currently I do this in a child theme’s style.css with…

‘.alignnone {
margin: 5px 20px 20px 0;
width: 100%;
}’

but I’m guessing there’s a much better way.

My site is running on localhost, so I cant offer a link to it.

Thanks in advance to anyone that can help. ??