This depends on which icons you are using. 

The Font Awesome icons are the most flexible. They will pick up the font size for the element they are in or you can set the font size on the <i> tag with the HTML Attributes tool.

The Canvas icons are an entirely different matter. Depending on how familiar you are with how CSS works, Canvas sets the font-size on the :before Psuedo element rather than using the element itself. That means that changing the font-size can only happen using a CSS stylesheet. We have done that work for the areas where icons are a default part of the functionality but that doesn't help if you are using icons in other areas.