Stand in the middle of a showroom with the tape measure in your hand and the first thing I'd tell you is to measure the room before the rug, not the other way around. Most people buy the size that looks right leaning against a wall, then get it home and discover it's floating in the middle of the living room like a bath mat. You want the front legs of your sofa and chairs sitting on it, at minimum, or it reads small no matter how good the pattern is.
A good rug changes with you over the years, which is part of the pleasure of it, though nobody tells you that at the point of sale. Wool will dull slightly and then settle into a softer, more forgiving color that hides life better than it did on day one, while a flatweave cotton or jute piece will show every spill for the first few months and then calm down once it's been walked on enough times to stop looking precious. Rotate it a quarter turn every season if it gets regular sun, because fading happens unevenly and you'll notice a rug shaped tan line otherwise. Vacuum with the beater bar off for anything with a loose weave, and get it properly cleaned, not just spot treated, every year or two depending on foot traffic and how many shoes make it past your front door. When you're checking one in the shop or the moment it arrives, get down on your knees and run a hand across the pile against the grain, you're feeling for consistency, not loose threads or bald patches, and check the backing for any smell of latex or glue, which tells you it was made fast and cheap. Fold a corner back too, a rug that won't lie flat on day one never will.
If you own nothing yet, start with a mid sized wool rug in a pattern busy enough to forgive crumbs and a dog's muddy paws but plain enough that it won't fight your furniture, because that's the piece that does the most work in a room that currently has none. Where a rug goes wrong is usually scale and texture at once, a thick shag piled into a small den where it eats the floor space, or a high sheen synthetic rug dropped onto a room full of worn wood furniture, where it looks like it wandered in from somewhere else entirely and never quite settled.
Keep it off direct sun as much as you can manage, and blot spills immediately rather than rubbing, which just pushes the stain deeper into the pile.