Apart from waiting:
Ways to increase your court size:
-Marry females of your dynasty (daughters, sisters) to your own courtiers, that way their children will grow up in your court.
-Be at war: you will get enemy claim holders in your court
-Deliberately keep the position of bishop unfilled, and accept the bishops the Pope pushes on you. This also works for other positions, except that the AI will fill these for you and you cannot unset them so usually the bishop is the only free one.
-Give your heir his own court, when your king dies and he takes over the two courts will be merged.
Bear in mind though that a too large court size will lead to disease events that can kill off your entire dynasty in the worst case.