Elle here from Shopify.
If you cannot find a font you like within the theme editor's general settings (Online store > Themes > Customize > Settings > Typography) then you can add more fonts by following this guide here.
You can modify the size of most text on your site within the Typography section as well; however, if your theme does not have the option to change the size of the product's price, it will have to be changed within the CSS of your theme.
Are you trying to change the price's font size everywhere? Or simply on the product or collection page? Let me know!