Guess it's a matter of opinion but I have used a 16" form. Tight squeeze but it worked for what I wanted to photograph. 10" - 12" is a comfortable fit. I personally don't care for the box and have made numerous changes to it. And, don't use the lights. Use 40W daylight bulbs. If I had a choice right now, I'd pick another light box. Be sure your worktop is high enough for the tri-pod. My worktop is too low and can't adjust tri-pod any lower. I rarely use it except under natural light.
White. This is such a great item! I take photos of my handcrafted pottery and the photos look great. The lights get really, really hot so I am always careful about turning them off or unplugging them. Highly recommend this product!