Well, as far as "How Big" or "How Long" or "How Much Weight" That's going to be trial and error (One of the reasons to have a release mechanism! ) Obviously some planes will pull a bigger banner that other, so your milage may vary.

Another thing to keep in mind is taking off...

You want to set the plane on the runway, then pull the banner out in front of the plane (But just slightly to one side so you don't hit it with the prop). This way, when you start to accellerate, you won't be dragging the entire load all at once.