A case screw for holding the cards is usually the same the holds the case sides on. Wether thumb screws or regular screws they are the same size.
Hard drive, and motherboard screws are smaller sizes. The odd case comes preloaded with a custom screw size and you have to use those. Some cases come with motherboard screws that have a hex head like the ones that hold the sides on but they are smaller so you can be fooled.
If you ever need a screw to hold a card in fast just steel one from the back of the case. If they are thumbscrews holding the sides on steel one of the 4 screws on the back of the case holding the power supply in. Those are all the same size.