It's hard to guess at it because we don't know how big the flowers are, but to me (just a guess) it looks like it's close to 1/4inch wide & since she's saying it's for a tween to adult, probably 17 1/2 to 18 inches.
I've been wanting to do a bunch of flowers on a headband too. One that I saw on etsy somewhere showed the back & what they did was attach the flowers to a piece of fabric that was cut to the shape that the flowers made when they were all together, and there was an identical piece of fabric under the headband. (The headband was in the middle of it between the flowers and the backing fabric, and the fabric was sewn around the edges.) I hope that makes sense.
That's what I plan to do when I give it a try. I've attached more than one flower to a headband, but mine were in a row on the headband, not extending beyond it. I'm still playing with it, but I also think that if it is thin fabric, it might need a stabilizer in between the fabric pieces so it doesn't flop around. Anyway, I hope that helps!