This will be determined by the sort of close-up photography you do. In general, the larger the room or topic in your photography, the larger the softbox would need to be. However, if you want to illuminate a small area with extreme lighting, a smaller softbox would suffice. Obviously, the illumination would be smaller and more focused on the subject with a small softbox for speedlight. A larger softbox, on the other hand, can provide much softer lighting, but it will need a larger light to fill it.