First and foremost, how large a sofa you can get depends on two things, 1. How large the room is, and 2. How large the doorway(s) is in which it will enter. There is nothing worse than ordering the perfect sized sofa only to realize it can’t fit through a door, into an elevator or around a winding staircase.
Again, if it’s just a sofa and not a sectional, you’ll want to make sure you can allow enough room for side tables, a coffee table, and lighting such as floor or table lamps. A room just is not complete without the proper elements that make that space function perfectly. Always carry your room measurements with you when you go out shopping for your large furnishings. If you find yourself questioning between the 6ft or 8ft sofa, you can always ask an associate or call an interior decorator or designer.
Once you have found the perfect sofa, make sure you can walk all around it, and can walk in between the edge of the sofa and the edge of the coffee table, minimum distance for that would be about 14”.
When you are thinking of adding other chairs to your sofa, note that people sitting in seats opposite to it should not be more than 8ft apart, as it is very difficult to have a nice conversation at that distance.