This is really simple, if you have the parts see if it fits.
If you dont have the parts, the company that makes the card (EVGA, Gigabyte, MSI, Saphire, etc) will have proportions on their website that says the size.
The case maker will have specs on the case that says the max size of card it supports.
We dont have 100s of cases and video cards at our disposal to check fitment on the spot, we just take the 20 second to do the research on it before we buy our own stuff.