The XSR is a new receiver soon to be released by FrSky. It looks to be primarily designed for the 130-200 sized miniquads, due to the only outputs being CPPM and SBus.
However this lack of PWM outputs does mean that it can be significantly reduced in size. It is 26 x 19.2 x 5mm, this is compared to the 40 x 22.5 x 6mm of the X4R and D4r.
Despite these advantages we are not very impressed with the use of a connector, we would have much preferred pads to solder to just like the KISS FC or Lux.
It is also important to note that it will only bind with X-series module, not the older style D series module. However if you are on a Taranis you will have no issues.