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.

mediasettingEnter the settings for the picture sizes in the boxes.

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.