The nuts are 13 mm square.
Not the average flat nut unfortunately..I ended up making my own out of 3mm stainless.
I found it impossible to buy them other than Roof Rack outlets but they are usually available up here from the 4x4 accessories places TJM and ARB.
Over time I made about 20 sets.
They were charging $3.50 each/nut up here a few years ago.