If you're standing there holding one, asking whether it's big enough, the answer is probably no, and that's fine. An evening bag isn't for carrying your life around, it's for holding a card, a phone, a lipstick and your keys, and if it does that without you having to force the zip, it's done its job. I've stopped trying to fit anything else in and started leaving things in the car or a coat pocket instead, which is the actual trick nobody tells you about.
The good ones age well because they're not doing much work. A proper evening bag spends most of its life sitting on a table or over a shoulder for a few hours at a time, so a stiff structured shape with a hard base holds up better than a soft slouchy one that starts to lose its form after a year of being squashed into a drawer. Look at what's holding the shape, a firmer frame or interior lining means it keeps its silhouette, while a floppy satin one will crease and never quite recover. Chain straps are more durable than cord ones, which fray at the point they meet the metal, so if you can, check that join before you buy, give it a gentle tug, see if it's stitched properly or just glued. Store it stuffed with tissue paper rather than flat, it stops the sides caving in, and keep it away from damp, because beading and embellishment pick up moisture and start to tarnish or come loose.
In the shop, check the clasp opens and shuts with one hand, because you'll often be doing it holding a drink. Check the strap length actually sits where you want it, cross body or on the shoulder, not banging against your hip at an awkward height. Check the lining colour against what you're likely to be hunting for in low light, a black lining in a black bag means you'll never find your lip balm again. For pairing, a simple structured bag in a neutral, black, or a soft metallic, goes with almost everything you already own, a slip dress, tailored trousers, even jeans with a good top, far more easily than anything with a strong print or a statement colour, which pins you into one outfit a year. If you've got none of these yet, start with that neutral structured one rather than the bold one you'll love for a single wedding and then ignore. Wipe hard surfaces with a soft dry cloth after each wear rather than letting anything sit on them overnight.