I've been making what i call "couch quilts" for a couple of years as wedding gifts....i am usually in the 50-56 x 70-76 inch range depending on the blocks i've chosen.

I like 60x72 ish. Big enough to quilt on my DSM without feeling like I'm wresting an elephant.

I just made a large throw and it was 54" x 72".

Usually 60x80. My husband is 6'5". I have 4 boys height ranging from 6' to 6'7". I try to make lap sized quilts long enough.

Make it easy on yourself. Have one of them lay on the couch and cover up with anything handy. Measure from the top edge under their chins to a few inches under their feet.

At least that is how I want my cover ups to fit.
And at least 4' wide.

If it's meant to be a sofa throw I like to make them 70 to 80 inches long but no more then 60 inches wide. I figure most people want to be covered with enough quilt so that their feet don't stick out when they move. Most of the quilt I make end up being twin to full size. Once in a while I make a queen size but only when I know who I will be giving it to. Usually then end up being whatever size I have enough material for. I rarely plan, usually go at it with no forethought whatsoever.