You can't expand fields, only shrink them, with the print/slide objects options.
Iy would not normally be best practice to print a layout with portals. You can let the user think that's what's happening, but it is more common to freeze the window, go to the related records table and show the correct related records (that appear in the portal), print that, then return to the original layout.
Then you can use the sliding controls the way you need, as well.
Thanks for the tip. Too bad. I've decided to start with expanding the field to two lines on the layout and seeing how that works. Can't get as many records on each invoice, but not many jobs require more than a dozen or so items, anyway. Perhaps I'll experiment with sliding controls later, when I have more time.
But no, I'm not printing layouts with portals. I have separate layouts for printing and they have layout parts and fields in the appropriate parts.
"But no, I'm not printing layouts with portals. I have separate layouts for printing" - from your Post comment: "I added the requested text to the relevant portal record and then previewed the results prior to printing" I assumed you were printing that layout, complete with portal. If you already have the other dedicated layout you'll have solved the printing problem. If you want to have the feature in browse mode, you can't; it only slides in print and preview.
If it only happens occasionally you could have a tooltip for the field that displays the whole field contents. You could highlight the field if it contains more than x characters (the number of characters that fit in the existing box).
Of course counting the number of characters in the field is only an approximate estimate of what will "fit" given that different letters have different widths unless you go "retro" and use a non-proportional font such as Ye Olde Courier. You might want to just set the tool tip to display the entire field contents for all numbers of characters.
A trick that I have used on a layout to squeeze a bit more text into view is to use conditional formatting to specify a smaller font size when the number of characters exceeds a certain number.