There are certain limitations for the TextBox control:
The text displayed by the control is contained in the Text property. By default, you can enter up to 2048 characters in a text box. If you set the MultiLine property to true, you can enter up to 32 KB of text. The Text property can be set at design time with the Properties window, at run time in code, or by user input at run time. The current contents of a text box can be retrieved at run time by reading the Text property.
If you need to have more text displayed then perhaps you should try using the RichTextBox Control.
Good Luck