A bedside lamp earns its keep in a room where someone reads before sleep, or gets up at odd hours and needs light without waking the whole house. If your nightstand is already crowded with a glass of water, a phone charger, and whatever book you started three months ago, think about scale before anything else. A tiny bedroom with a tiny nightstand does not need a lamp with a shade the size of a drum kit, no matter how good it looked in the shop.
When you're looking at one in person, or unboxing it the day it arrives, check the switch first. Is it on the cord, on the base, or up near the bulb socket where you'll be fumbling for it half asleep. That one detail decides whether you curse it every night or barely notice it. Check the weight of the base too, because a lamp that tips over when you knock it with an elbow at midnight is a lamp that eventually meets a wall, or a glass of water, or both. Over time, the finish is what tells you whether it was worth the money. A good metal base will pick up a few scuffs and look better for it, like it has a history. A cheap painted one chips at the edge within a year and starts looking tired fast. Fabric shades yellow near the bulb eventually, that's just what happens, so wipe them down with a dry cloth now and then rather than waiting until they need it. Cords fray at the point where they bend near the plug, so don't yank, and don't wind the cord too tight around the base when you move house, which is exactly when these things get dropped, trodden on, or left behind entirely because nobody wrapped them properly.
For sizing, measure from the top of the nightstand to where your eyes land when you're sitting up in bed. The bottom of the shade should sit roughly at that height, not above it, or you get glare every time you look over. Too tall and it dominates the whole side of the bed, too short and you're reading by a strange underlit glow. Leave a few inches of nightstand showing around the base so it doesn't look like it's about to fall off the edge, and measure the nightstand's width before you fall for a wide shade that will only fit on one side.
I've swapped out more of these than I'd like to admit before landing on the right height and switch position for my own bed. Get both right and you stop thinking about it, which is really the whole point.