Cloud is not a clone of Zack. If he was a clone of Zack, he would look exactly like him. He would also have the traits of Zack, and he probably wouldnt be a grunt sentry, he'd be a full on SOLDIER. Not to mention that if these kids were experimented on at a young age, they would have been taken from the parents and raised as SOLDIERS, to make them more effective. There's no evidence that these kids were ever tampered with before they lef for Midgar when they became teenagers.

Zack and Cloud being around the same height and body type is just coincidence in the game, neccesary for the makers to allow Cloud to take Zack's uniform and fit into it perfectly.