The following example shows how to create it:
Dim CATDocs As Documents
Set CATDocs = CATIA.Documents
Dim part1 As Document
Set part1 = CATDocs.Add("CATPart")
Dim Width As Length
Set width = part1.Part.Parameters.CreateDimension("width", "LENGTH", 20.)
By default the units are the default units of the IS of units.