I would like to share a good article on 4/3 sensor
Interesting pics taken from www.wrotniak.net
(Viewfinder Size Difference)
Interesting summary by wrotniak on the sensor:-
To sum things up, size-wise at least:
1.All three most common types of DSLR sensors (APS-C, Foveon/Sigma, Four Thirds) have practically the same size; therefore any discussions of differences between them due to the size are groundless and just demonstrate that someone did not bother to check the numbers.
2.The full-frame 35-mm (or 135-type, using the old film terminology) sensors are roughly twice that size (linear) or four times (area).
3.The smaller (2/3") sensors are approximately half that size (linear), or 1/4 of the area (the difference between those and 1/1.7" or 1/1.8" is, again, insignificant).