Increase Length and Width of Boxes

I have a form display on our contact us page. One of the boxes on the form is labeled “your message”, so users can send us an electronic message — exactly like I am doing right now. I would like to increase the length and width of that box to allow the user more space to type. The default setting is for the box to be very small, which makes the user think they do not have room to type a lengthy message. How can I increase the length and width of the boxes?