Hi,

I don't recommend you do that because if you do that, all your images can be scale and it's too bad for a website. If you want to make the images inside your paragraphs center, you can use this custom css code:

\
.entry-content p img { display: block; margin-left: auto; margin-right: auto; }

For change the image size when you insert images for your posts, please click on the image on media library > and select Image Size on the right side - check this image.

Best Regards,

PenciDesign