Which Railway station is biggest among other stations in India?

Kharagpur 

howrah station is the biggest railway station

Kharagpur

On what basis " On longest  or on the basis of number of platforms"?

Anyhow Mughal sarai is one of the biggest plat forms of India.More details can be had from this link

http://www.walkthroughindia.com/walkthroughs/trains/top-6-longest-railway-platforms-india/

I suppose the Howrah station.

Howrah station in Kolkatta. It is one of the biggest station in India. It has 26 Platforms.