Do you want to change the default size of the pictures i.e thumbnail, medium and large size pictures of your WordPress blog
Click on “Setting” and then on “Media” on the left side of the dashboard.
If you installed “WP Smush.it” plugins for compressing the picture size then you can select the automatic setting for compressing the picture upon uploading.
After finishing all the settings click on “Save Changes”
This is how you change the default size of pictures in WordPress blog.