The honest answer to "will my stuff fit" is that most small crossbody bags hold four things, and you have to decide which four. Nobody writing product copy wants to say that, so they say "surprisingly roomy" instead and let you find out when the box arrives.
Here is what actually goes in one.
The four things
A phone. A card holder or a slim wallet. Keys. One small extra, usually a lip product or a compact.
That is the working set for a small evening bag, and it is the set the bag was designed around. Anything past four items and you are either sitting on the zipper or carrying the fifth thing in your hand all night.
Where phones ruin the math
Phone size is the variable that decides everything else. A standard phone sits flat against the back panel of a small bag and takes up almost no depth. A Pro Max or an Ultra with a thick case is a different object: it eats the width, it pushes the front panel out, and it turns a bag that held four things into a bag that holds two.
Before you buy any small bag, measure your phone with its case on. Then look at the bag's stated width. If the phone is within about half an inch of the bag's width, it fits but it dominates. If it is wider, it does not go in at all, or it goes in diagonally and stops the zipper closing.
What a wallet costs you
A folded billfold with cash, cards and receipts is roughly the same volume as a phone. Two phone-sized objects in a small bag means nothing else fits.
The fix most people land on is a card holder. Four or five cards and a folded bill takes maybe a fifth of the space a real wallet does, and for a night out you are not using the loyalty cards anyway. If you carry a full wallet daily, keep a separate card holder that lives in the going-out bag, already loaded. It removes the transfer step that makes people leave the small bag at home.
Keys, and why they are the worst item
Keys are not big, but they are hard, they are sharp, and they move. In a lined bag with no interior structure they scratch a phone screen and dig at the lining from the inside.
Two things help. Put keys in an interior zip pocket if the bag has one, which keeps them separated from the screen. Or carry a single key on a small ring instead of the whole set. A full keyring with a car fob and six keys is genuinely a quarter of a small bag's capacity.
The interior pocket matters more than the size
Two bags with identical outside dimensions hold very different amounts depending on whether the inside is one open cavity or a cavity plus a zip pocket.
An open cavity means everything slides into one pile at the bottom and you dig. A zip pocket sized for cards means the flat items stay flat and stay findable, and the main space stays free for the phone. It is a small feature that changes the bag from annoying to usable.
Our pearl quilted crossbody has one interior zip pocket sized for cards, which is where the keys and the cards go, and it leaves the main compartment for a phone and one extra.
What does not fit, in any of them
A water bottle. A laptop or a tablet. A full makeup bag. A hardcover book. A large camera. A folding umbrella, usually.
If your night involves any of those, a small crossbody is the wrong tool and no amount of careful packing fixes it. That is not a flaw in the bag. It is a bag built for an evening where the whole point is not carrying much.
A packing order that works
Load flat items first, standing up against the back panel: cards, cash, anything thin. Then the phone, screen facing the back panel so the hardware on the front of the bag is not pressing against it. Then keys into the zip pocket. Then the one extra, dropped into whatever gap is left.
Loading in that order takes the same items and makes them close comfortably. Loading in reverse order makes the same bag feel too small.
Before you buy
Check three things: the bag's width against your phone's width, whether there is an interior pocket, and whether the strap actually adjusts. Everything else is taste. Those three decide whether the bag gets used twice or gets used all year.
If you want the short version: assume four items, make one of them a card holder instead of a wallet, and measure your phone first.