You need an empty USB stick and a .iso disk image for this.
Goto System > Admin > Startup disk creator. Once you've selected the source and the target, you should be able to select how big a persistance file you want to get at the bottom of the dialog. This stays grayed out for me forever.
Does it work for you? If you're using something else than Trisquel 5.0, be sure to mention it.
