No, its not true.
The maximum sender window size depends upon two factors:
1) No. of frames that can be tranferred by sender in RTT.
2) No. of bits available for sequence number field.
Thus maximum sender window size is, Ws = min( 1+2a, 2N).
where N is the no of bits available.
So, in a case where 2N is less than 1+2a, the statement is FALSE.