Scaling edge thicknesses

Update: Could not get CurrentValue[..., "ImageSize"] to pick up the correct image size.

Based on Sjoerd's update, an alternative approach would be to post process a graphics object to transform the thickness primitives using the aspect ratio, plot range and image size of the input graphics. This can be done in a number of ways using AbsoluteOptions (getting PlotRange or AspectRatio) to calculate a new thickness.

Sjoerd's example (slightly modified)

 grpSjrd1 = Graphics[{FaceForm[], EdgeForm[{Thickness[0.1], Blue}], Disk[{0, 0}],
     Disk[{3, 0}], Disk[{6, 0}]}, ImageSize -> 300]
grpSjrd2 = Graphics[{FaceForm[], EdgeForm[{Thickness[0.1], Blue}], Disk[{0, 0}]},
    ImageSize -> 300]

ClearAll[thicknessUnits]; 
thicknessUnits[gr_Graphics, sc_] :=
 With[{units = 
       AbsoluteOptions[gr, AspectRatio][[1, 2]]},
  gr /. EdgeForm[{beg___, Thickness[thcknss_], end___}] :> 
     EdgeForm[{beg, Thickness[sc units ], end}]]

Usage examples:

Grid[Transpose[
  {thicknessUnits[grpSjrd1, #],thicknessUnits[grpSjrd2, #]} & /@ 
      {.03, .05, .1, .2}], Spacings -> {3, 2}]

Alternatively, one can use the PlotRange instead of AspectRatio:

ClearAll[thicknessUnitsB]; 
thicknessUnitsB[gr_Graphics, sc_] :=
  With[{units =-Subtract @@ AbsoluteOptions[gr, PlotRange][[1, 2, #]] & /@ 
      {1, 2} // (#[[2]]/#[[1]]) & },
   gr /. EdgeForm[{beg___, Thickness[thcknss_], end___}] :> 
   EdgeForm[{beg, Thickness[sc units ], end}]]

---

Leftover from original post ... (holding for now until I give up struggling with CurrentValue[...])

Using CurrentValue["Magnification"] seems more promising:

Column[Style[Graphics[{FaceForm[],
  Dynamic@EdgeForm[{Thickness[.02 CurrentValue["Magnification"]], Blue}],
  Disk[{0, 0}], Disk[{3, 0}], Disk[{6, 0}]}, ImageSize -> 600], 
Magnification -> #] & /@ {.2, .5, 1., 1.5}]

Perhaps you are looking for Inset?

disk = Graphics[{EdgeForm[{Black, Thickness[0.1]}], LightBlue, Disk[]}];

Graphics[{
  Inset[disk, {0, 0}, {0, 0}, 1],
  Inset[disk, {2, 0}, {0, 0}, 2],
  Inset[disk, {4, 0}, {0, 0}, 1.5]
}]

Show[%, ImageSize -> 200]