Limiting anything is either hard or impossible if the application is
intended to allow end users to do what they want.
In this case, you could consider enabling the Control toolbox and inserting
2 fixed size textboxes with some spaces in between, and setting their
properties so that "multiline" is true. Then switch out of "Design mode" and
save the document.
Problems include:
a. IME using controls this way in a Word document is slow and crash-prone
b. you may not be able to prevent your users from adjusting the size of the
textboxes, or deleting them, anyway
c. there's no automatic text flow between one control and the next.
d. users just get to type in the font you specified, with right, left, or
centred formatting. No other formatting, no justified text etc.
Peter Jamieson