How to change font size on "Proceed to Checkout"

Could you give a link to the page ? (the CSS will depend on your template so we can't give you an answer without checking your template CSS and it's effect on hikashop's CSS)

You will find the font-size in the file www.capsulespeaker.com/Clarity/templates...ity/css/template.css of your template in the class button.

However, the font size is already 11px. You won't be able to go a lot lower. You will probably have to change the text in order to make it smaller : "order" instead of "proceed to checkout". You can do that in the config under the tab languages by editing your translation file.