15mm, for the nut on top.
Disconnect the battery.
Disconnect the starter cables, makes access it easier to remove and refit.
The bottom ones are 13mm. Do these first
Using a 15mm 1/4 socket with an extension or two, make it so that the ratchet EDIT: bar is longer than the starter if possible.
Undo the nut while pulling the starter away from the bell housing, this way the socket does not get trapped.
Refit as they say is a reverse .
Cheers