The max window size could be N=1+2a
[it means 100% efficiency]
Bandwidth of T1 trunk=1.544 Mbits/sec
Ref-https://en.wikipedia.org/wiki/T-carrier
Transmission Delay($T_{t}$)=$\frac{Length-of-Frame}{Bandwidth}=\frac{64*8}{1.544*10^6}=0.331 ms$
Propagation delay
for 1 KM distance the propagation delay is 6 μsec
for 3000 KM propagation delay=$\frac{6*3000}{10^6}$=18 ms
$a=\frac{T_{p}}{T_{t}}=\frac{10}{0.331}=54.3$
1+2a=1+2*54.3=110
No. of bits in sequence number=$log_{2}110$=7