hi maneesh,
i looked at your structure, and it looks ok. in particular, both electrodes have the same length. they might still be too short though.
then, i reproduced your steps, took the central region and clicked Device from Bulk. I manually set both electrodes to 7.0 A.
what you see at the bottom of the window are not the electrode sizes but steps in which you can increase/decrease the central region size, i.e. here you can shift the electrode-central region
boundary. the fact that these steps are different from left to right electrode indicate that the code has difficulties in actually finding good matches based on the periodicity in the z direction.
this is also the reason why you do not see any predefined electrode lengths in the electrode length combo box.