I used to work in a large format newspaper (broadsheet) and the smallest we were allowed to go was 3pt and that was on A1 page size which formed a double page spread. I would work from that, as I can't read anything from print when it's under 5pt. How are you going to print it as that's a lot of page tiling if your doing it yourself. Even with large format printers the biggest one that I have seen was 8 feet across the bed being fed by a roll. If you want the family tree to be read by your family I would scale your page size by at least 12.