1)How can I ensure that all fields have the same length ?(see the attachment registration fields alignment)

Edit your fields and adjust the Size parameter as needed. This only works for fields that support it though. Date field for example can't have its size changed.

2)How can I Show the profile completeness field at the top of the page for registration?

You can't, no user exists yet during registration so there's nothing to check against.

3)How can I make a step wise registration process?(which shows the progress bar at each stage)

You can have Step by Step registration by enabling it within CB > Configuration > Registration. There is no progress bar for this though and you'd need to develop such a usage.

We don't have multi-page registration. The best you could do is enable step by step registration, but it's still all on one page. The alternative is to figure out how to implement something like that. The progress bar could probably be done with some simple jQuery using CB Auto Actions and its Code action on the registration display trigger. You'd write some jQuery to utilize jQuery UIs progressbar widget then determine the completeness based off change and keyup events fired for various inputs.