Now you can easily increase the size of the HTML checkbox. You just need to apply CSS3 properties.
Here is the CSS code to add with your CSS class or ID.
Example:
input[type="checkbox"]{
-ms-transform: scale(2); /* IE */
-moz-transform: scale(2); /* FF */
-webkit-transform: scale(2); /* Safari and Chrome */
-o-transform: scale(2); /* Opera */
}