Hi Francesca, indeed at the beginning a bit bigger board is good, but you do progress very quickly in winging so the board size can be reduced normally once you are getting gybes/tacks more regularly and feel comfortable. Actually the size depends also on the local conditions you'll ride in - so keep the 5'8" until you are up and moving on the foil, maybe someone can help you with some pumping technique advice for the board and the wing, to get up.
And then depending on your conditions, either a 5'4" or a 5'0" - 5'0" I would say if you have regular winds, should be big enough. If you are mainly on a Lake with very light winds/altitude, then you might be better off with the 5'4", but that is quite a big board still for your weight.