By default, the Sketch toolbar contains tools to create various Sketch to Scale markups.
If the toolbar is currently hidden, go to View > 
Toolbars and select it to show it.
Polygon Sketch to Scale: Adds a polygon markup to an exact scale.
Rectangle Sketch to Scale: Adds a rectangle markup to an exact scale.
Ellipse Sketch to Scale: Adds an ellipse markup to an exact scale.
Polyline Sketch to Scale: Adds a polyline markup to an exact scale.