Guess how much space you need. Start off with a solid color background and then if you need more space just go to image>resize canvas (or something like that) and then make it bigger. Before hitting okay after resizing make sure you hit the top arrow out of the like 8 there are to choose from, this will make sure what you had will stay on top and everything added will go to the bottom.
All layers taken from a sig or w.e you are working with should have the same original dimensions.
As for the background you could get a premade one or make it yourself.