Large throw pillows make sense if your sofa or bed looks a little lost without them, the kind of seating that swallows up two small cushions and still looks bare. They work especially well on deep sofas, oversized sectionals, or a bed with a headboard that needs something substantial in front of it. If your couch is already on the small side, or your armchair is snug to begin with, oversized pillows will eat the whole seat and leave no actual room to sit down, which defeats the point of having a chair.
The honest limit with these is that they take up space whether you want them to or not, and most people end up fighting them nightly, tossing them on the floor before bed and wrestling them back onto the couch every morning. If you've got a small apartment living room or a reading nook where every inch of seating matters, a large pillow is more furniture than accessory, and it will not read as cozy, it will read as clutter. They also show their filling fast. Cheap inserts flatten into a sad pancake within a few months, so a pillow that looked generous and plush on day one ends up looking like a deflated cushion that's given up. A better insert, feather and down or a dense fiber fill, holds its shape and bounces back after someone's leaned on it all afternoon, which matters more than almost anything else about the whole purchase. Linen covers soften and relax with use in a way that looks intentional, cotton holds its shape but wrinkles the way a shirt does, and velvet looks rich but shows every handprint and crease under certain light, so it suits a formal living room more than a den where people actually flop down. These pillows earn their keep most in bigger, slower rooms, a living room meant for sprawling out on a weekend, a primary bedroom with a bed big enough to need the visual weight, a sunroom in the cooler months when you want something to curl into. They're less suited to high traffic rooms where kids or pets are constantly moving through, since bigger pillows get knocked around more and show wear faster simply because there's more surface to get worn.
For colors, I'd steer toward something in the oatmeal, clay, or deep green range rather than anything bright white or heavily patterned, since a plain neutral large pillow is easier to live with daily and doesn't compete with everything else in the room. Over time, the best ones soften at the corners, lose that showroom stiffness, and start to look like they actually belong to your house rather than a store shelf.