Since I have one I thought it would be easy to look up in the owner's manual, but it only describes them as "fixing bolts" without giving the size of the bolts. Their long with an allen wrench head, without dismantling mine, I want to say maybe 1/4 inch thread and they require a washer too. Sorry I can't be of more help, but I would just try a few sizes and see.

Here's a page from the manual.