When you're trying to determine the size of a rug for a room, I think, you want to think about how you're using the room and how your furniture is placed in that room. For me, if you're looking at a space, I'm trying to define that space, which is the furniture around it. I'm not looking to, basically, put it in like wall-to-wall, I'm looking to just define that space, or what I call the area.
For a living room, I think you could do lots of ways, but my way of looking at it is I would prefer to put the furniture on the rug, just not all on the rug but, at least, the legs of the furniture on the rug, to help anchor it and also help to define the space and make it more cozy and inviting as far as the space goes. I think the legs, the front, being rested on the rug helps keep the furniture from sliding away.
For a bedroom, I think a little differently about bedrooms. I think a bedroom is one big space. I tend to prefer to do one big rug, and there I would probably look at the room size and try to do it within about, let's say, 18 inches to 2 feet, somewhere in there, around the perimeter of the rug and the wall space. That's usually the way I would personally want to do a bedroom but I think there's other ways of doing it.
Another way would be to define those open spaces. Usually, there are three spaces in the bedroom alongside of the bed which is longer, skinnier. We call it a runner type size or a scatter type size. Then usually, a bigger rug, maybe a 5 x 7, 6 x 9, at the foot of the bed in that open space at the foot of the bed. That could also be proper or very nice. It's a little more challenging when you're mixing different designs and colorwise when you're doing three rugs, but you could also do three of the same rug. To me, it's boring, but you can definitely do that as well.
A dining room is pretty simple to me because I think you want to have enough rug in order to slide back your chair. What I always do is measure the top of your table, but you about 99% of the time measure that size and add 5-6 feet to that measurement. So basically if your table is 3.5 feet (42 inches) which is pretty standard, by 50 or 60 inches, which is 5 feet -- if you add 5 feet, that'd be a 10. If you add 6 feet that'd be 11. I would say, either an 8 x 10, 9 x 12 is a very common or size we'd recommend for a dining room because you don’t want to not be able to scoot back.
Now, there are situations where you have to be aware of your wall space. You don't have that much space to put a rug in that size. I would say, at that point, you get whatever you can as big as you can in there, but I would say, that 6 ft. range is really what I would try to add to your tabletop. It’s what I'd like to see as optimum.