It varies between gates and even between individual times the gate opens; the arms move around into different configurations, leaving open different amounts of passable space. The accessible space is, on average, enough for at least two people walking abreast to go through carrying decent sized packs, though often it's not advisable for more than one person to come through at a time. One of the difficulties of gatecrashing comes from this - you have to break everything larger down (vehicles, buildings etc) to component parts, physically carry them through and reassemble them on the other side. Some gates can be quite large - the gatecrashing book mentions some being 30 meters across - but it doesn't give dimensions of the Fissure gate specifically. It's up to you, really.