My experience has been that the Line Width dropdown in the Line Gallery does not allow you to type in a number (and has been this way since around XDP version 6 or so), however the Line Width dropdown on the main toolbar *will* allow this. I've had to train myself to use that box instead.

That said, XaReg will allow you to edit the contents of that and other dropdown lists (such as font size), so you can add 5 and 6pt or whatever.

-- Ben