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. 

The only thing I can add here to the otherwise excellent responses is that rears should be used mainly for ambience and are not meant to carry the load.  Monitor size speakers should be adequate for this.  TBH I have never heard a system that uses floorstanders as rears, so I can’t really justify my claims with experience 

I have full size speakers for front and rear.wih dsp my denon avr 110 has the microphone that evens it all out.atmos is great.some music recording now are in atmos.they have self prowered subs.with class d 1500 watts each.class d is where it's at now for base speakers.the home theater stuff now is better than the theater.16 Chanel sound who needs stero.that should get it going.enjoy the search.with the dsp it's able to control time delay gain ie volumn  lessing the effects of different amplifiers.alot of these processors for home theater have xlr out the back so you can use internal amp or shut that channel off and use external amp.this makes it better for the front and center channel that require more wattage than the rest of the surrounds.you can even set it for stero mode only

I have full range spears at each of the corners (FL/FR/RL/RR) which was a decision made to support the multichannel SACD format which does spec full range rears.

With that said, I have never once wished that I had smaller speakers in the rears.

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. 

@pcolvin - In the mid 1980s I worked in theaters as well as a major manufacturer of electronic equipment and racks for theaters.  I am going to call you out and say state your sources.

I’ve been behind at least a dozen different theater screens and can categorically say that the main LCR speakers were always full range and identical.  Surround speakers were usually different in large part because they had to be mounted up about 12’ high and were quite visible.  Surround speakers were therefore size limited and there were limitations in the high frequency content in the original Dolby Surround tracks, but we can think of them as full range. 

In addition to being behind the screen I also helped calibrate a variety of commercial movie sound processors, my experience ends before Dolby Digital however.  None of them had built in bass management to enable a subwoofer.

The subwoofer you say was always there was most certainly not.  It wasn’t until the limited use of Sensurround in around 1971 that there was EVER a subwoofer in a theater.   And while that was an important achievement it wasn’t until Dolby Digital that a separate low frequency effects (LFE) or bass management was even a normal thing in theaters.  As far as I know the standard is still for full range speakers behind the screen AND subwoofers.  The use of small main speakers is strictly a home thing.

The other weird thing you state is that they were limited at 200 Hz.  No idea where this comes from.  Even when THX started branding satellite speakers as THX certified the THX spec was you used a sealed speaker that went down to 80 Hz. Any higher than that and the bass would be directional.  The sealed requirement was to ensure proper phase and amplitude matching with the subwoofer.