Part document. Look for GetRange:
Public Sub GetRange( _ ByRef MinRangePoint() As Double, _ ByRef MaxRangePoint() As Double _ )
If it is an Occurrence look for Range method:
Public Sub Range( _ ByRef x_min As Double, _ ByRef y_min As Double, _ ByRef z_min As Double, _ ByRef x_max As Double, _ ByRef y_max As Double, _ ByRef z_max As Double _ )
This is only valid if part is aligned to the default coordinate system. Oherwise values are the bounding box that contains the part and it could be bigger that the part dimensions themselves.