Textarea row height seems overriden by theme CSS preventing it to take actual rows height. So this can be fix by adding simple custom CSS. Follow the steps below:
1. First you need to Go to your Dashboard -> Appearance -> Customize -> Theme Options -> Custom CSS options.
2. In this section you need to add this CSS:
.wpcf7-form-control {
height: auto;
}
You can adjust no. of rows from CF7 shortcode.
Write us if problem exists.