Probably a 14x7.00 or 14x7.50 according to the sales brocure:
http://oldcarbrochures.com/static/NA/Plymouth/1963%20Plymouth/1963_Plymouth_Brochure/1963%20Plymouth-10.html
http://oldcarbrochures.com/static/NA/Plymouth/1963%20Plymouth/1963_Plymouth_Brochure/1963%20Plymouth-10.html