The dimension is 6.5’ minimum. That is a clear space measurement. The measurement is not to the disconnect (some panels do not have a main disconnect). I can find nothing that specifies either a minimum or maximum for location of the panel itself, but it must be readily accessible.
For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm.
a. Equipment, piping and ducts foreign to the electrical installation shall not be placed in the shaded areas extending from the floor to a height of 6 feet above the panelboard enclosure, or to the structural ceiling, whichever is lower.
b. The working space shall be clear and unobstructed from the floor to a height of 6.5 feet.
c. The working space shall not be designated for storage.
d. Panelboards, service equipment and similar enclosures shall not be located in bathrooms, toilet rooms, clothes closets or over the steps of a stairway.
e. Such work spaces shall be provided with artificial lighting where located indoors.