Ahoy Jean,

I have always used 7/16" on both my 3/4oz and 1 1/2oz tri-radials.  I have had no failures with these.  Both the guys and sheets are low tech, New England Ropes "Sta-Set X".  Newer generation, smaller, lighter weight, rope would work also and may be preferable in the most extreme lightest winds.  I recommend that you prepare any line that may become a Pole Guy with good chafe protection where the pole jaw will ride.  I also use spinnaker shackle guards that ride between the pole jaw and the attachment of the rope to the clew, whether that attachment be a shackle or bowline.  (my guards are plywood "donuts" and I still tie bowlines).

Good Luck,     Dave