<p>Is it possible to set the size of a checkbox using CSS or HTML across browsers? </p>

<p><code>width</code> and <code>size</code> work in IE6+, but not with Firefox, where the checkbox stays 16x16 even if I set a smaller size.</p>

My understanding is that this isn't easy at all to do cross-browser. Instead of trying to manipulate the checkbox control, you could always build your own implementation using images, javascript, and hidden input fields. I'm assuming this is similar to what niceforms is (from Staicu lonut's answer above), but wouldn't be particularly difficult to implement. I believe jQuery has a plugin to allow for this custom behavior as well (will look for the link and post here if I can find it).