Hi Patrick,
Zinnias stretch to higher than their "advertised" heights when they are planted too close together, creating competition for light, and thus extra height. Right off hand, I think the tallest packet description zinnia height is Benary's Giants at 4 feet.
" I just need for next year a much Shorter Zinnia. "
There are many varieties of short zinnias, but since you are replacing your zinnias with Iris rhizomes, this height question is becoming moot.
ZM
