Digital Camera Database published an article comparing mirrorless camera sensor sizes. As can be seen from the infographic, sensor sizes of mirrorless cameras vary even more than those of DSLRs:
Olympus and Panasonic are commited to Four Thirds system, while most other big companies use slightly bigger APS-C sized sensors. Nikon took a different approach and went with considerably smaller CX sensors.
You can read the whole article here.