You don't need spurs for showing, or for riding at all necessarily - but your choice of spur, its rowel type and shank shape/length is going to be governed by the length of the rider's leg compared to the horse's depth of girth, how sensitive the horse is and the rider's leg control.
They are also a very personal thing - I'd be inclined to get a voucher from a Western store rather than try to buy spurs for someone else.
