In my opinion, the icon is too large at 24 x 24. I am using the Windows Classic theme. I used the following CSS in my userChrome.css file to resize it to 16 x 16:

.stackalert-button *
{
    width: 24px !important;
    height: 16px !important;
}

In my opinion, the icon is too large at 24 x 24. Other Firefox toolbar icons are 16 x 16. I used the following CSS in my userChrome.css file to make it the correct size:

.stackalert-button *
{
    width: 24px !important;
    height: 16px !important;
}