This depends on the size of your forklift. However we would recommend at least 1000mm clearance on the width and 500mm on the height, so for example, if the forklift is narrower than a standard pallet which is 1000x1200mm then make the door a minimum of 2200mm clear opening, and if the forklift is 2500mm high with the mast in the loaded travelling position then the doorway should be 3000mm high.  It is safest to double-check with your fork truck supplier.