Photo tent /soft box sizing

I was wondering if anyone had any experience with the soft boxes / photo tents. I am kind of confused on the sizing. I need to photograph the eyeglasses/sunglasses, and need the correct size box. I was initially planning to get a 12" one, but now it turns out that it would be too small probably as some of the reviews suggested.

Any suggestions on what to get?

I use a 19", you could have more use and you can place sunglasses in the center with less edges showing if doing darker exposures. Taken by a Canon G12, RF-602 wireless trigger and just 1 430EX II speedlite positioned on top outside the tent. Just experiment yourself what you want to achieve. Hope it helps