Agree with what's been said above. Just to summarise:
The standard size of desktop harddrives is 3.5"
The standard size of laptop harddrives and SSDs is 2.5"
Many newer desktop cases have both 3.5" and 2.5" bays because SSDs are very popular in desktops. If your desktop case does not have a 2.5" bay you can buy a 3.5" to 2.5" adaptor for under $10.
So yes, all harddrives will fit in all desktop cases, but you may need to buy an adaptor if you buy an SSD. (Or do what I did, and velcro it to the bottom of your case. No moving parts, so SSDs can be shoved anywhere).