By nature, climbing roses will shoot straight upwards and then flower at the top, leaving a lot of usually uninteresting leggy stems. To avoid this, encourage them to grow in a series of horizontals, so that flowering starts much lower down and the arch is more fully dressed. I’ve described how this is done in more detail, with lots of further tips, here. When deciding on the dimensions of your arch, remember to take into account the roses’ bushiness once they are established, which will reduce both the width and height under the arch.