(I do not understand what you mean by "line drawing form" or "the image" in the context.)
There is no way to define a truely relative line width relating to whatever size property, and no tool to scale the line width on resizing.
If you urgently want such scaling, you will need to create user code. This would not be exactly simple. To do auomatic scaling while resizing with the mouse may be specifically difficult and would most likely not result in reversible processes.
A more practicable way might be to set a new (absolute) line width relating to some overall size of a shape as it currently is. Problems of a different kind will arise if you want to apply the new method to GroupShape or ShapeCollection objects.
A first sketch of user code doing code as decribed for a single ungrouped shape:
Sub setLineWidthRelativeToDiagonal()
tSC = ThisComponent.CurrentSelection
last = tSc.Count - 1
j = -1
relLw = 1 * InputBox("Enter theRatio:", "LineWidth / TotalSize", "0.01")
' You are prompted for the input in advance of testing if it is applicable
' to any selected shape.
Do While j<last
j = j + 1
tSh = tSC(j)
If NOT tSh.SupportsService("com.sun.star.drawing.Shape") Then Goto notApplicable
If tSh.SupportsService("com.sun.star.drawing.GroupShape") Then Goto notApplicable
With tSh.Size
dX = .Width
dY = .Height
End With
totalSize = Sqr(dX*dX + dY*dY)
newLw = totalSize *relLw
resizeRat = newLw / tSh.LineWidth
tSh.LineWidth = newLw
tSh.LineStartWidth = tSh.LineStartWidth*resizeRat
tSh.LineEndWidth = tSh.LineEndWidth*resizeRat
notApplicable:
Loop
End Sub