I understand that some of the phone users out there are very particular with phone sizes. Here is an comparison of the size of an S3 and S4 side by side comparison. You can even place your phone on screen to compare. I have scaled the image accordingly for users out there to compare their phones on a print out. Just for the fun of it.
I purposely dont label them. Can you tell the difference?
S4 has a more elongated length as compared to S3. It is also 15% thinner than S3. The material color is definitely pure white as compared to the yellowish tinge of S3.