Anyone know the dimension of this nut?

Anyone know the size of the nut circled in red, in this photo?



I lost it on my serpentine belt tensioner and I'm hoping I won't have to change all of the tensioner. If I can find a similar nut at an autoshop, I should be able to repair it right? If all else fail, I'll try to find one used tensioner in a scrap yard and I'll use the nut from this tensioner.

That should be a bolt and it is an 8mm and it is about 20mm long