Sorry, Joan, I can't cite any firm guidelines. Perhaps another member has some working lengths that he or she can offer for reference.
I recently did a landscape that was about 38 inches wide. I think that's too long a span for only a standard stretcher bar (or "strip"). There's simply too much deflection in the center of the bar when you hank down on those canvas pliers. But in this case I was unable to use heavy-duty strips because the clients already knew what frame they intended to use, and the extra-thick profile of the heavy-duty strip wouldn't have worked aesthetically, so I chose to simply add a cross-brace. {I haven't delivered the painting yet, but I've been informed that the client, while demonstrating the intended location for hanging the painting, dropped the antique frame and damaged it beyond repair.]
Certainly by the time you're working with dimensions of 3 or 4 feet each way, I think you want to be using heavy-duty stretchers. That's roughly the size of this
portrait (the posted image is cropped), on which I used both HD stretcher strips and a cross-brace -- perhaps excessive, but I was using a heavy double-primed Belgian linen and I knew I was really going to lean on those pliers to get that canvas stretched, and then I wanted it to stay there. I knew I would be making my own frame and that it would be quite wide and deep, so the thick profile of the HD strips wasn't a problem.
Besides considering the deflection over a long span, I just look at the size of the proposed piece, realize that I'm going to ask the stretchers to support a canvas drawn tight as tympani heads over that area, and ask myself if the standard size looks and feels strong enough. Not very scientific, I know.
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