If you want to allow multi-line input and have the width expand to accommodate the number of characters on a line, you need to set:
• AutoSize = True
• Multiline = True
• WordWrap = False
and either set:
• EnterKeyBehaviour = True
or rely on Shift-Enter to insert new lines.

One problem you may experience is the textbox scrolling the top line out of view. Scrolling back to the top should fix that.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]