On a base 69, would the rally wheel size be 14 or 15's? I have a 69 on thevway that is on "spares" so wheels and tires will be one of to buy items.

 

Only Z/28's & COPO's came OE with 15 inch wheels.
One good check is the glovebox decal - tire size is stated.

Check trunk lid inside for Limited Slip decal. If one is present, then full size spare, unless option was ordered from original assembly.