Plugs are designed to protect the LED. To quote India from the sales team for a question about blade plugs for thin necked saber: "The main purpose of a blade plug is to protect the LED face the thin center emitter area [of thin necked emitters] is protecting the LED face from scratches already which is why a blade plug would not be required." So it's primarily protection for non think necked sabers (the emitters will indicate if they're thin necks in their notes, like the prodigal son or 08 emitters). Thin necks apparently don't require plugs since they already protect the LED. And they may not fit properly in thin neck emitters (though that may be solved with cutting the plug down, depending on the thin necked emitter).
It also helps limit the strength of the LED when there's no blade in it so you don't blind people with the strong light.
Beyond that, it's decoration to make it look like there's a port on the emitter that the beam comes from, rather than a bland hole.
Since your emitter isn't a thin necked one, a blade plug will be helpful to protect the LED. And I'd say a 1" or 1 1/2" would be sufficient for the emitter you listed. But you can specify in the order notes which emitter the plug is for and they'll probably make sure you get the right length.