Width and indeed, all measurements are measured by twips.

FROM THE ACCESS 97 **** HELP *** FILE:

Put your cursor on the "width" part of your above code and simply press
F1 and you'll get a section on:

Height, Width Properties

Look about halfway down the page, under "Setting" and you'll find:

"In Visual Basic, use a numeric expression to set the value of each
property. Values are expressed in twips."

Since you probably won't look at help anyway or claim you don't have it
or can't get the above, "twips" is linked to something curious which
tells you that what you're missing above is (assuming you're using inches):

Me!AddnlOwnrList.Width = 5.1875 * 1440

Are you trying to set the list box width to 5.1875 inches?
Access property sizes, i.e.  Width, Top, Left, etc. are all measured
in Twips (1440 per inch).
A width of 5.1875 Twips would be so small as to be un-viewable.
Try:

Me!AddnlOwnrList.Width = 5.1875 * 1440.