What is the size of an octagon soft box

I'm thinking of getting a large octagobox, and I'm looking at this one:

https://www.amazon.com …liid=I3HDQ9HOMF​X6EJ&psc=1

My question is, what is the 70 inches measured from/to? Is that the diameter (seems way too big), or is it the circumference?

Should be the diameter. Most use point-to-point rather than flat-to-flat as it makes things sound bigger. A few (PCB's PLMs, for instance) measure the same distance but following the back curve of the octabox such that the true straight diameter is less than that quoted.

They're typically measured by diameter. The reviews in the link describe it as "big" and "huge". You should get an answer soon from someone who has first-hand experience with this one.

Its the diameter.