I am assuming that you are using FileMaker 11. FileMaker 12 starter solutions are quite different, so let me know if you are using FM 12.
The add a note script could be modified to either switch layouts to a layout where you have a larger size version of this field or it could use the new window script step and open a new window and change layouts to this same layout with a larger notes field.
Then you'd need a pause in your script so that the user can enter the note and click a button with the "resume" option to continue your paused script.
You could also add a button to the portal row that uses Go to related Records to open the record in the row on this same layout--also in a new window if desired. This could be a way to view and/or edit a selected record.