First of all thank you so much for your response, I appreciate it.
So I asked wether saying sender window size = no of acknowledge bytes at that instance is correct as you explained.
I got this doubt because I thought that sender could have any amount of data to send as it depends on application program, but turns out that amount of data sender could send constrained by window size is referred as “unacknowledged bytes at the sender” and this resolves all the quiries. Thanks again!