Mike here from Shopify. So quite easily achieved, but where to look? I use the trusty Google Chrome Dev tools to figure this out :)
So any element on a page can be right clicked on and inspected with this tool. The icons in your inquiry are no different. The icons are controlled by the CSS ".site-header__link .icon", which is located in your theme.scss.css file. Defaulted with a size of 22px x 22px I went and changed mine to an exaggerated 100x100 for purposes of this example.
Finding your theme code is easy too : ONLINE STORE>THEMES>3 Dots..>Edit HTML/CSS. Then scroll down to the assets directory you will see the theme.scss.css file there. Click on that and if using a MAC for instance %F to search the file. locate .site-header__link .icon and you should see the sizing present. Change to desired size of "px" and save, and your theme should now have larger icons (in that spot) as a result.
If you find this a little overwhelming though, or not comfortable with code. Always know that you can reach out to a developer at https://experts.shopify.com, or because this is a Shopify supported theme and if you are on a plan, you have some design time allowed, make the request with any one of support channels, such as Chat, Email or Phone : https://support.shopify.com/ and a guru can put you in touch with our theme team.
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