Hi raghav-jaya, if you mean the blue whale - like the one in our Mammals gallery - they grow to be about 25 m (male) or 26 m (female) as an average length. To put that into perspective, it's a similar length to the Diplodocus skeleton in our Central Hall, or a bit longer than two double-decker buses end-to-end.
However, they weigh considerably more than the Diplodocus (which is estimated to have been about 20 tonnes when it was alive). Blue whales typically weigh somewhere between 90 and 120 tonnes, but the heaviest recorded weight was a whopping 190 tonnes for a female caught in the Southern Ocean in 1947.
P.S. I am moving your message to the Your Museum Visit forum.