Ariane,
I had a similar need except that I only had one field in the group that could grow. But you should be able to modify the script below to do a comparison first to find the tallest fields. I use the following javascript in the Exit event of field that can grow:
var ht = xfa.layout.h(this,"mm");
var ht = ht+"mm";
//app.alert(ht);
this.parent.itemNum.h = ht;
this.parent.objClass.h = ht;
this.parent.costCode.h = ht;
this.parent.Qty.h = ht;
this.parent.Unit.h = ht;
this.parent.unitPrice.h = ht;
this.parent.Recd.h = ht;
this.parent.totPrice.h = ht;
I also have a reset button that clears the form. I had to manually reset all the row fields to the correct height in the click event of my reset button. Good luck!