Enter layout mode.
Click on the portal (not the memo field)
Drag a selection handle down to make the portal row taller.
Now click on and resize your memo field in the same manner, but keep the borders of the memo field inside the borders of your portal row. You may need to open portal set up and specify a smaller number of rows so that this portal does not extend to far down on your layouts.
With the memo field selected, you can also specify a scroll bar for the text field in the Inspector's data tab as well. This way, if your portal extends past the bottom boundary of the field, you can scroll the field to see the remaining text.
The above changes are for data entry. If you need to print out your memos, a different layout with sliding fields can be created that allow fields sized very large to shrink down to just the size of the data in the field.