Is Full-size speakers for L-R Rears advisable?

The original surround sound standard does not include a need for FS speakers for anything other than LR.  All other speakers were 200hz and up. This has been carried through for decades now in the cinemas. Bass has always been handled by sub-woofers. 

@erik_squires I was the engineering manager for Litton-Westrex until 1983 (when it was sold to Quad-8) which did sound-on-film since the Jazz Singer in 1929, 6-channel film recorders, and record cutting lathes/cutting heads. 

At the time I worked there I was involved with the early days of Dolby and DTS for the theatre, and my engineering staff included Keith O. Johsnon (who created the dbx Sub-harmonic synthesizer which was used to drive subs for the Pacific Cinema Dome in Hollywood) and Terry Beard (of DTS). 

In partial answer, at the time Altec-Lansing A-1’s with the multi-cellular horn was used behind the screens (depending on the size of the theatre you had LR or LCR) and was run from a 35W/chl tube amp, and for Dolby Surround Electro-voice was used (various models) for the surround channels with Ray providing the upgrade to the amplification chain.  In the theatre world if you want to put out something new you have to pay for it yourself: Universal Sensaround was similar in case as Universal paid for the equipment and installation to have it put into the various theaters and removed it afterward.  Subs were not added until the time of Dolby and THX.

Sounds like you worked around the Hollywood area.  Outside of Hollywood, most of the theaters were 35mm optical with A-1s behind the screen and, again, some 35w tube amps. No equalization was done beyond the original Academy curve, which was provided in the amps:

Key Characteristics of the Academy Curve:

• Flat response: The curve is flat from 100 Hz to 1.6 kHz. 
• Reduced bass and treble: The response is attenuated (reduced) at lower frequencies (40 Hz, 7 dB) and higher frequencies (5 kHz and 8 kHz, 10 dB and 18 dB respectively). 

Hence, no need for subs.

Additionally, remember that at that time you were, and still may be, shooting sound through a fabric screen with a bunch of tiny holes in it.  The Academy curve took this into account as it also took into account that you were laying sound tracks onto the film between the sprocket holes and the film frames. 

Back in the 80's Ray Dolby, Tomlinson Holman, and Terry Beard had to prove to the Academy and the theatre owners that changing sound quality would result in increased sales. It took a lot of time, experimentation, and show & tell before what we have today caught on. Terry's use of separate time-synced CDs/DVDs under the DTS (Nuoptix) name brought digital audio into the mix, whereas before Ray had to deal with a encoded stereo and 4 channel optical mixes (unless it was a 70mm release).