在模态对话框中设置文本框控件的大小

Firstly I'm fairly new to the coding scene so please so easy on me. I have an issue where I am putting a text box control inside of a modal dialog window however I cannot seem to get the textbox to resize to take up the full size of the modal window. It just seems to take up half the width of it.Firstly I'm fairly new to the coding scene so p