Changing the Media Sizes will set the image sizes for future media uploads. If you want to change the sizes of images that has already been uploaded, then you need to regenerate image sizes.

If you want the images to have the same sizes/proportions as in the demo, then you need to setup the media sizes. These are set automatically when importing demo content, but you can also set them manually here:

Blog and Page media sizes

These image sizes are set automatically when you import the demo content. If you want to make sure that you have the correct settings, navigate to Settings → Media and set the following image sizes shown below.

You can change the maximum height, but I recommend keeping the widths as specified below:

 

WooCommerce Products

You don’t have to set image sizes for WooCommerce products, but you can set the proportions/ratio. If you want the the same proportions as in the demo, navigate to Customizer → WooCommerce → Product Images and set this custom ratio:

After uploading your own product images, you can play around with different Custom aspect ratios here – or select Uncropped to keep the original aspect ratio of your product images.