I think as long as it is shorter than the width of the eyes of your beardie, it's okay because Dubias don't have that much chitin, I guess. But you should wait for the more experienced guys and gals here.
I would wait until your beardie is 16-18" before feeding them adult dubia. Their digestive tracts are pretty much mature by then. 13" dragons are still pretty small.
How much does your beardie weigh?
Our dragon was about 18" and weighed around 250g before we started giving him large prey. Adults have a harder shell than nymphs do, and takes some powerful chomps to get through.
thanks! I decided no to. I'm not going to risk it . He does eat the 1 inch-ish nymphs without a problem, so I'll just stick with those for a few more months.