If you are looking to cover the difference between the one line you can see and maybe 3-4 other lines, then I make the tooltip for that field the contents of the field ('Self'). Then when I mouse-over the field I can immediately see if there is any more text, what it is, and if it is worth clicking on to see more detail. (If it can run to a long list than I make the field on the portal list a button that takes me to that record in detail, where I can see it all.)
You can also reduce the number of rows in your portal and then resize the portal row so that you can resize the memo field to be three lines of text tall and include a scroll bar on the field if you wish. While portals default to a "table like" view of the related data, you are not limited to that option. You can actually resize the portal row and rearrange/resize the fields within the portal row to get results much closer to a "mini-form" than a "mini-table" if you wish.