You can limit the number of characters fairly easily:
Enter Manage | Database | Fields, find your field's defintion and double click it.
On the validation tab, you can enter a calculation such as Length(self) < 20 to limit the number of characters entered to 19 or fewer.
Text entered into FileMaker can start a new line simply because the user typed in enough characters so that text wraps to a new line or it can be due to pressing the return key while entering text. You can disallow returns by either setting the field's behavior in the Inspector so that pressing return moves the cursor to the next field or by using this auto-enter calculation:
Substitute ( self ; ¶ ; "" )
Also clear the "Do not replace existing values..." check box.
The auto-enter calculation option will remove returns even when the text is pasted into the field.