leafletjs marker bindpopup() with options

You can pass an object of popup options as the second argument of bindPopup, like this:

marker.bindPopup("<strong>Hello world!</strong><br />I am a popup.", {maxWidth: 500});

I've tested this in Leaflet 1.4, and it also seems be available in earlier versions of bindPopup.

Are you sure that you're reading the Leaflet reference documentation? It specifies that you can bind a popup with options by creating it and calling .bindPopup with it. For instance,

var popup = L.popup()
    .setContent("I am a standalone popup.");

marker.bindPopup(popup).openPopup();