DAWN theme - Image resize on product page

Okay since I’m not a computer wiz, can a person just go to the section-main and change the size and that will take care of the entire store, or does a person have to go to the inspect on each product page and do something first? What about an image on the front page of the store, if they want it smaller? Will going to the section-main and then change the size in there take care of all pictures? Just trying to figure this out as the pics are to big and slowing my store down. Thanks in advance for helping me because I’m not quite understanding everything on the video. It went to the inspect on the page and then it went to the section-main, so I’m not sure if a person needs to do both to make it work.

Marilyn