This plant, also called Mexican Petunia, comes in Blue, pink and white. Most of my experience is with the purple-blue variety. It grows up to 5' tall and 3' wide but can be kept with pruning at 3' by 2'. This variety has a tendency to spread and send up new plants from the roots, but I have found that it can be controlled with regular pruning. The best place to plant it is in a contained area, such as an island bed which is surrounded by asphalt or an edged bed. Then you can just let it go. Planted in a mass these plants are a gorgeous splash of blue all summer and into the fall. They take to hard pruning so if they get too tall just hack them back to 2'. They will be flowering again shortly after.
Combine them with red canna, shrub allamanda for a beautiful combination. When combined with other plants you may have to keep up the pruning as they are aggressive for their space.