If initially sender has window size equal to cwnd = 2MSS then sender will continue with it until congestion detected by -
1) time out occur, then start with slow start phase.
2) 3 ACKs then start with congestion avoidance phase.
In case 1 Sender's threshold will be reduced to half of the current window size $SSthreshold = \frac{1}{2}cwnd$ and again congestion window size will be set to 1 ie $cwnd = 1$.
In case 2 Sender's threshold will be reduced to half of the current window size $SSthreshold = \frac{1}{2}cwnd$ and congestion window size will be set to sender's threshold ie $cwnd = SSthreshold$. It then start congestion avoidance phase.
For more reference refer
https://gateoverflow.in/2156/gate2012-45
PS :
http://inst.eecs.berkeley.edu/~ee122/fa06/notes/18-Congestion-2up.pdf