Rules of Soccer

From the Federation of International De Football Associations (FIFA) Laws of the Game, Guide for Referees, July 1993.
"Goal-posts and cross-bars must be made of wood, metal, or other approved material as decided from time to time by the International Football Association Board. They may be square, rectangular, round half round or elliptical in shape."
"Goal-posts and cross-bars made of other materials and in other shapes are not permitted. The goal-posts must be white in color."
"The width and depth of the cross-bar shall not exceed 5 inches (12 cm)."
From the National Federation of State High School Associations' (NFSHSA) 1994-95 National Federation Edition-Soccer Rules Book.
"They shall consist of 2 upright (posts) 4 inches but not more than 5 inches (0.10m by 0.12m)...the tops of the posts shall be joined by a 4 inches but not more than 5 inches (0.10m by 0.12m) horizontal crossbar..."
From the National Collegiate Athletic Associations (NCAA) Rules for Soccer.
"...and shall consist of two wooden or metal posts, ...the width or diameter of the goal-posts and crossbar shall not be less than 4 inches (10.16 cm) nor more than 5 inches (12.7 cm)."