Sketcher > Dimensioning Sketcher Geometry > Working with Dimension Tools > Perimeter Dimensions > To Create a Perimeter Dimension
  
To Create a Perimeter Dimension
1. Click Sketch > Perimeter. The Select dialog box opens.
2. Select the chain of entities you want to dimension and click OK in the Select dialog box.
3. Select a varying dimension. The perimeter dimension is created, the varying dimension value is followed by “var”.