Hi, and welcome to BigBlueBall, Saurabh.
I just tried an IM to myself, and it wouldn't let me type past a certain number of characters. That exact number appears to vary depending on what I actually type, between around 765-815 somewhere (I may have miscounted by 1 or 2!).
And when sent, all that was sent was received ok, so I guess that would be the answer !?!