I would think that the length of the bolts would be determined by the thickness of your heel block. Since the heel block is roughly 40mm I would say you should use a bolt 40mm. Any longer and the bolt would bottom out past the tenon on the neck. 

Just to confirm - the bolts supplied in the standard kit are indeed 6x40mm.

You are welcome @koendb. Mark did use a 6mm bit. I personally just found the bolt I was going to use and then just measured the shank and picked a drill bit that worked. The one I found in my part of the world had a larger shoulder before the threads so I needed to use a slightly larger bit. Whatever gets the job done. 🙂

Thank you both of you for sharing the information , will apply this.