Before purchasing a bifold door, it is important to know how bifold doors are sized. Stated bifold door sizes are in nominal increments. Similar to regular doors, the sizes are expressed in the finished opening size. For instance, a 3' 0" bifold door fits inside of a 36" wide finished opening. The leaves of the bifold are appropriately undersized to allow for clearance around the door.
The same is true of the height of the door. Bifold door sizes are based on having a track at the top and a pivot bracket at the base. Generally, you need an extra 1 3/4" of height beyond the height of the door panel. The bifold doors are undercut 1" standard. For a nominal 6' 8" opening, you need a finished height of 6' 8 3/4" (80 3/4").
Translated that means a 79" door plus 1 3/4" for the track and brackets. Since this height is sometimes a problem, you can get bifold doors that are 2" shorter. The doors are 77" plus 1 3/4" equals 78 3/4". This will work for openings that are 80" finish or slightly less.
Another thing to keep in mind, about how bifold doors are sized, is that bifold doors are sized in pairs. A 2' 0" bifold door will have (2) 12" leaves that are held together by hinges. Again the spacing of the panels is allowed for in the nominal size.