For years I bought throws the same way I bought socks, whatever was folded nearest the till, no thought given to size. The result was a drawer full of things too small to do anything with except sit on the arm of the sofa looking decorative and useless. What fixed it was treating a throw like a piece of furniture you have to measure for, not an accessory you grab on the way past.
For a sofa, you want enough to drape properly, which means more than you think. Lay it flat and it should be close to the width of the sofa back plus enough drop on each side that it hangs rather than perches, so for most three seaters that is somewhere in the region of a generous square, not a slim rectangle. For a bed, the same rule in reverse, it needs to clear the width of the mattress with enough left to pool at the sides or fold at the foot, otherwise you end up with something that looks like a bath towel someone left behind. The checks you can do with your hands before you buy anything, pick it up and see how it falls when you let it go, a good one drops and settles instead of standing stiff. Hold it up to the light too, if you can see straight through it, it is going to pill and thin out fast.
Where it goes wrong is pattern on pattern, a textured throw over a patterned sofa or a heavily patterned bed with a loud throw on top just reads as noise, so one of the two surfaces needs to stay calm. It also goes wrong in a small room with low ceilings, a huge chunky knit on a small sofa can swallow the whole piece of furniture and make the room feel like it shrank overnight. And a throw is not a blanket substitute for actual warmth on a cold night, the lightweight decorative ones are for the look and a layer, not for keeping you from reaching for the heating.
If you own none, start with something mid weight in a plain or barely there texture, in a color that sits a shade off your main furniture rather than matching it exactly, because exact matches tend to disappear and barely there contrast is what makes a room look finished rather than planned. Avoid anything too precious to wash, life with a throw on the back of the sofa means it gets used, pulled over feet, sat on, occasionally rescued from the dog.
The detail that makes a good one worth having is weight at the edge, a proper hem or a bit of fringe that gives it enough body to hang in folds rather than flop flat, so it still looks deliberate three weeks after you stopped arranging it.