I am having a trouble with this waza. Most of time, I just end up with blocking instead of excuting Kaeshi men. It seem it is harder to excute againse 1) tall kenshi, 2)Fast kenshi, 3) Big men motion.

Should I step back? What is your problem with this waza?

Response from a 7.dan...
Try changing to doing a harai type of attack. Don't just receive their shinai but attack it near the tsuba and then hit men. If the person is a hard charger and going through you will not be successful very often.
You can move slightly to your right with hiraki ashi but it really works best on those that are one step type of attackers. If your mind is waiting you will always be slow and late. You must have an attacking mind-seme in order to do these kind of waza.

Who is the right target kenshi to use such waza then ?
1) One step attacker
2) Not too tall
3) slower kenshi

Follow up from 7.dan...
The height is not that important. One of my 5-2 gals uses this waza on 6 foot guys all the time. She has fast hands and feet, so if they are one steppers or slower she gets them. She moves to her right, hits the opponents shinai on the left side near the first knuckle so as to gain maximum deflection and done with the timing of kote-men (only faster) like clapping your hands twice very quickly. You move to the right ever so slightly on the second hit.
Don't move your feet on the first hit, the opponent is already doing the footwork for you. I teach this to all my students, about 50% can use the waza. As with all waza the person has to want to use it in order to master it. Trying it a couple of times will not bring success and you must know when to use it. It is a very good counter to jodan.

How far in on her own shinai does she hit?

Follow up from 7.dan...
You want to be close to the tip of your shinai so that you make contact at the earliest moment in the evolution of the waza. This gives you the time to make the second hit on the men.

Regarding the footwork you can also pivot on the ball of the right foot and step diagonally back to your right with the left foot to gain a little space. The pivot and step are done as one motion. Haga sensei used this method to do men-kaeshi-do. This changes your angle of attack slightly to give you a better line on the men for those faster folks that you need a little more time to affect the waza against.