Recently when creating families in Revit, I came across a problem. When creating a square extrusion, lets use for this example a table top. If you want to have curved edges on the front and back edge of the table, it is possible to draw your extrusion in an elevation view e.g. Elevation left, as shown below. What you will achieve is, as you would expect a rectangle extrusion with rounded edges as you have drawn the profile in the elevation.