by checking your site with firebug, it looks that the 16px width and height of the category icons are not set by any style rule, but are hard-coded in the corresponding template that renders the icons - probably the template for the side-menu which is 'customer/categories.tpl' (you can find the exact template by using the webmaster mode), so you might want to look into it for code similar to
Code:
<img width="16" height="15" alt=""
which is repeatedly rendered on every anchor element, for each category, so the above code will be located inside a {section}...{/section} or {foreach}...{/foreach} pair of smarty tags for looping the rendering of each category and icon.