The thing nobody tells you about wallets is that the one you fall for in the shop is almost always slightly too big for the bag you actually carry day to day. It looks neat on a counter, then it goes into your bag and suddenly it's the thing you have to move to find your keys. Worth sorting out anyway, because a wallet that works properly saves you a small daily fumble at the till, the card machine, the bus, and that adds up over a year more than people realise.
Fit here is really about your bag and your hands, not your body, but the same logic applies: check it properly before you commit. Take whatever bag you use most and see if the wallet sits flat in it or pokes a corner out, because a wallet that's a few millimetres too deep will always end up wedged at an angle, and that's how corners wear through first. Open it one handed if you can, standing up, the way you'd actually do it at a till with a queue behind you. If the card slots are stiff when new, they won't loosen much, they'll just fray. Check the stitching at the corners of the coin section if there is one, that's where cheap ones give up first, usually within a few months of coins going in and out. Press the clasp or popper shut a few times in the shop. If it feels like it's fighting you on the fourth go, it'll feel worse by October.
The small things worth checking before you pay are the ones you won't think about until you're annoyed by them at home. Does the zip pull sit flush or does it catch on your coat lining. Is there a slot that actually fits a travel card or an Oyster without you having to tip the whole thing upside down. Does the leather, or the leather look, have any give to it, because a wallet that's rigid on day one tends to crack rather than soften. And count the card slots against what you'll actually carry, not what you imagine you might carry, because half the complaints people have about wallets being "too bulky" are really just too many slots left empty and flapping.
Where this goes wrong is if you're someone who's moved almost entirely to a phone case with a card slot, in which case a proper wallet is dead weight you'll stop carrying within a month. It's also the wrong buy if you need somewhere for a lot of receipts or loose notes, most slimmer ones simply don't have room and you'll be cramming.
The one mistake to avoid is buying for the inside of the shop rather than the inside of your own bag.