How do I manually add size on product images

Step: 1

In your shopify Admin panel, under THEMES & SETTINGS section in the left sidebar menu, click on the "Theme setting"

Like this:

Step: 2

After this, all theme settings page will appear. In the bottom left section "Product images" section available.

Like this:

Step: 3

 In this section you can change product images size by changing image ratio  to "Manual".

 

You can manually add width and height of the images.

Like this:

 

Step: 4

Once you done with all settings, Click on the "Save settings" button.

Your settings will be saved successfully.

That's all done.