What pots for what flowers

When you buy seeds on the back of the packet is written the size a plant will reach at maturity, but I was skeptical until I saw it with my own eyes. Now I know and the size of the plants for the right pot are as follow.

8 to 12 inches plants in pots 6 inches size.
12 to 18 inches plants in pots 8 inches size.

The only problem with 6 inches size pots is that you need to water daily in very hot weather(in texas reaching 107 F) and it may become tedious. That's why using smaller size pots I consider practically nuts!.

Zinnia Zahara rose in 6 inches pot.

You can use rectangular pots, circulars, whatever shape you want, especially if you want to give an accent to a garden area