You could employ a simple userform with two listboxes and a command button. Let prone to user errors:
Code:
Option Explicit
Private Sub UserForm_Initialize()
With ListBox1
.AddItem
.List(.ListCount - 1, 0) = "0.25 pt"
.List(.ListCount - 1, 1) = 2
.AddItem
.List(.ListCount - 1, 0) = "0.50 pt"
.List(.ListCount - 1, 1) = 4
.AddItem
.List(.ListCount - 1, 0) = "0.75 pt"
.List(.ListCount - 1, 1) = 6
.AddItem
.List(.ListCount - 1, 0) = "1 pt"
.List(.ListCount - 1, 1) = 8
.AddItem
.List(.ListCount - 1, 0) = "1.5 pt"
.List(.ListCount - 1, 1) = 12
.AddItem
.List(.ListCount - 1, 0) = "2.25 pt"
.List(.ListCount - 1, 1) = 18
.AddItem
.List(.ListCount - 1, 0) = "3.00 pt"
.List(.ListCount - 1, 1) = 24
.AddItem
.List(.ListCount - 1, 0) = "4.5 pt"
.List(.ListCount - 1, 1) = 36
.AddItem
.List(.ListCount - 1, 0) = "6 pt"
.List(.ListCount - 1, 1) = 48
.ListIndex = 3
End With
With ListBox2
.List = ListBox1.List
.ListIndex = 5
End With
lbl_Exit:
Exit Sub
End Sub
Private Sub CommandButton1_Click()
Dim oTbl As Table
For Each oTbl In ActiveDocument.Tables
oTbl.Rows.Last.Range.Select
With Selection.Range
With .Borders(wdBorderTop)
.LineStyle = wdLineStyleSingle
.LineWidth = ListBox1.Column(1)
.Color = wdColorAutomatic
End With
With .Borders(wdBorderBottom)
.LineStyle = wdLineStyleSingle
.LineWidth = ListBox2.Column(1)
.Color = wdColorAutomatic
End With
End With
Next oTbl
lbl_Exit:
Exit Sub
End Sub