First of all, I'm not sure if 5-ft octabox is the best option for your shoot. There are many ways to light a full length shot, and they produce different looks.

If you are shooting a fashion catalog or lookbook type work, Id rather use two small lights rather than one big one. I have Elinchrom Octa 74-inch and I love it for certain things but I prefer two 45-inch or even 30-inch lights over Octa 74-in for this type of work. If you use an incident light meter you see the illumination is even with Octa, but the way shadows and highlights show up is not as nice. In my way of seeing things, huge softboxes are a lot more useful as a fill source. If you use big light as the key, you can easily get dull images.