No firm rule, but it should be properly proportioned. For example, this sofa was 90" wide, and we positioned a sofa table that extended just until the sofa back began to curve. In this case the top was 62" long. 48" would have been too short. This picture doesn't illustrate the sofa table we used. This one illustrated is too tall and too long.

Thanks!, yes right now they are just small plantlets, separated them and I am sure once they grow bigger and taller....the look the display will be ever changing. @razamatazzy...the sofa, we bought a few months ago at Macys for around $600, its the Chloe velvet tufted sofa. It feels lovely and doesnt stain easily. The reviews are really good, so far I have had no problems, though its only been a few months for me. Cooks.

Instead of a drum shade you could choose a tapered shade with a narrow opening at the top. There are some lamps that are completely closed at the top, usually with metal or glass shades (probably because an enclosed shade could get too hot with a high wattage bulb for other materials). There are lots of Tiffany-style table lamps with dome or closed tapered shades, if that suits your style. Another option would be to get a tall lamp with a tall shade. Something with this kind of proportions: Another option would be to use a swag lamp rather than a table lamp. It gives you a bit more flexibility in placement, and might open up new opportunities.

Thanks everyone, definitely some good ideas here. I'm reluctant to put anything large into that space, like a screen or a tall shelf, because I think it will block the openness of the room (which I mostly like) plus the ceilings are so tall it would be hard to find anything proportional for that narrow but tall space. For now I'm going to play with the lamp and art/mirror placement, hoping to find something that will visually balance a bit better. I'm hoping Carrie B and auntthelma are right, that the right focal point on the wall will make the gap less noticeable, instead of trying to fill the gap with something. (Though I have thought about putting a table behind there, like Nira Juso and Nicole Shields Design suggested, I just need to find something narrow enough that it doesn't block the entryway to the bathroom.) I also like the idea from housegal200 to put a mirror over the couch to visually open up the wall instead of filling in the gap, as I do sort of think the fireplace is the natural focal point, and I don't want every surface to compete with one another. Negative space is important, I'm just not sure that gap is my ideal negative space :) I'll try to come back and post a follow up once I've got my next arrangement. Thanks again everyone, and if you have more ideas please do keep posting.