How to interpret the window size and FFT size while performing FFT? I have a discrete signal of length 98,000 samples and I am supposed to a FFT of the discrete signal with certain window size and FFT size. Can someone help me explain what is meant by the window and the FFT size here? 
Also, what would the size of the output of the signal post FFT?

Thanks in advance!
There are lots and lots  of related questions on this topic in this SE and also on many discrete signal processing books. 

https://www.sjsu.edu/people/burford.furman/docs/me120/FFT_tutorial_NI.pdf
https://holometer.fnal.gov/GH_FFT.pdf