I would cut it back to below the start of the new growth but not by too much. This will keep it nice and bushy.
Depending on how cold it gets will make a difference as to how much it dies back. Our Winters have become much milder to the point where even non-hardy varieties are surviving outside. This means they are not knocked back so much. You may only need to cut it back by half or 2/3rds. Some of the branches may die back completely in which case they would have to be cut back to the crown (5/6cm from the base) The fuchsia would resprout from that point.
It all depends on how exposed it is to the elements and how cold our Winter gets this year. Too many variables to give you a hard and fast rule.