Default location for the classic wow is C:\Program Files (x86)\World of Warcraft
Check the DATA file: Size on disk 4.02 GB (4,325,068,800 bytes) according to windows 10.
If you have retail some how sneaking in, it will be 50 to 60 some odd GB, blizzard chose to merge classic and retail data but they are called up by different .EXE and that determines what is used. If some how the retail data has managed to get into the data file you can uninstall the retail data by opening the launcher then with the little up or down arrow you can select the version, then 3/4 the way up the left side of the UI you will see option, then uninstall, it will prompt you to confirm then you can delete.
You can back up all your macro, keybinds and all that stuff if you like by copy and paste the whole C:\Program Files (x86)\World of Warcraft_classic_ Folder, it contains all the settings but none of the game data, its also got the EXE, but no matter because the file is very small like 260mb or something.
If you are still missing space, do a drive clean up, some times temp files eat up a lot of space.