Sorry for the late reply, for some reason i didn't get an email notification of this thread.
The skin doesn't come with Fixed and Fluid width XMLs, it is already set to responsive by default. It will only expand to a certain point and only go down to a certain point that won't break xenforo.
The reason they don't come in XMLs is because adding support for 5 XMLs and managing them is a hassle to everyone. As a result you can manage the width by going into the AdminCP > Appearance > Styles > Dragonbyte MMO > Templates > Extra.css find and modify the following to suit your needs: