Geoff,

My Strega kit (big model, 700++ sq. in. almost 3 inchs thick at the root) uses 1/16 inch ribs-----but many of them.  Something like two inch centers, I think.

With cap strips, rib thickness may be less critical.

Cheers,
Jim

As noted the ribs are there just to give the airfoil shape.   1/16" is good for just about any size airplane AS LONG AS there are capstrips.  The capstrips turn the rib cross section into an I-beam and contibute to the "hangar handling" strength and help control warping of the wing and the ribs themselves from covering. The bending strength of the wing comes from the spars and shear web and in most designs the LE sheeting.

A few years ago I built a Cardinal and had nothing but trouble with the wing wanting to crush from handling as well as being very weak in bending.  There was basically nothing left of the ribs vertically due to the cutouts for the leadouts.  Instead of putting in full shear webs I put 1/8 sq strips between the spars at the ribs, and 1/8 x 1/4 diagonals in place of shear webs between the ribs.  The strength of the wing was immensely improved.  I finally pancaked it in inverted, pilot error, and the wing did break in half but it was due to overzealous sanding of the center section sheeting.  Nothing happened to the rest of the wing.