It's not at all apparent how you expect to have 11pt text in tables with a 6pt row height, but they're your tables:
Code:
Sub FormatTables()
Application.ScreenUpdating = False
Dim oTbl As Table, oCel As Cell
For Each oTbl In ActiveDocument.Tables
With oTbl.Range
With .Font
.Name = "Arial"
.ColorIndex = wdBlack
.Size = 11
End With
.HighlightColorIndex = wdYellow
.ParagraphFormat.Alignment = wdAlignParagraphLeft
.Cells.VerticalAlignment = wdCellAlignVerticalTop
For Each oCel In .Cells
If oCel.Height > 6 Then
oCel.Range.ParagraphFormat.Alignment = wdAlignParagraphCenter
End If
Next oCel
End With
Next oTbl
Application.ScreenUpdating = True
End Sub
The above code processes all tables in the document. To process a particular set of tables, change ActiveDocument to Selection, then select the tables to process before running the macro.