You do not need to find it. You just have to come up with it. Any words that are known to you are written by Latin, small letters without spaces and specials.
You can use the code without it, in this form
.leaflet-popup-content {
width: 1000px !important;
}
But this code will have an impact on any other Open Street Map addons that you may want to use in another place of your site and where you will not need a width setting.
Adding your own class to Addon allows you to make the code isolated, so that this would have an impact on other addons.