Lucio - tell us the actual size of the opening. Standard door (from stock) heights come in 6'6" and 6'8", so your 'missing' couple of inches could be because your frame is made for the bigger standard door.
All bi-folds are 6'6", actually the doors themselves are shorter than this because the dimensions of the track are taken into account. You might be able to get one made to order.
If you have to use a bi-fold, say 'cos the entrance to the bog is tight, then one option could be to fashion a 'header' piece to the frame to lower it to fit a 6'6"er; you might have to re-work the architraves. I wouldn't, unless you're skilled, attempt to heighten the door.
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