Thomas M. Olsen wrote:

A question has come up in regard to the correct sizes of the lettering
on Reading box and other freight cars prior to the introduction of the
"Speed Lettering" in 1954. Can anyone advise as to the correct sizes of
the road name and data lettering?

Let's separate your question into two parts:

1) The size of the RDG name and logo, and
2) The size of the stenciled data.

Rules governing the size of the stenciled data were in most CAR BUILDER CYCLOPEDIAS in the "Freight Car Construction: AAR Standards section." According to that section in the 1940 CYCLOPEDIA,

1) Roman Letters & Figures were to be used.
2) The sizes of the letters & numbers were to be confined to 1", 2", 3", 4" 7" and 9".
3) 9" lettering was to be used for initials, names (Flat Cars could use 7" if space was a constriction.).
4) 7" for car numbers on the sides of cars.
5) 4" lettering for reporting mark and car number on the car's ends.
6) Symbol of classification of car (AAR Mechanical Designation) - 4".
7) Capacity, Load Limit, Light Weight, New or Station Symbol - 3".
8) Date Built, IL, IW, IH, Cubic Capacity, Couplers, Markings on Inside of Car, Markings on Trucks to be 2 inches.
9) Repacked data, Air Brake Cylinders or Reservoirs as per Interchange Rules 60 and 66 - 1" in 1940.
10) Trust Marks, Patent Marks & Other Private Marks - 1".

I'll let the RDG experts answer the size of logos the RDG used prior to 1954.

Tim Gilbert