The secret to parametric families are reference planes. Of course, you can apply parameters that adjust the dimensions of elements without the use of reference planes, but some things just won’t work that way.

One of those things is filled regions. A good example of this is an electrical distribution board symbol. Distribution board symbols are always shown dimensionally accurate (Revit detail item), unlike a GPO for example which would always appear the same size on a sheet of paper regardless of the scale (Revit annotation symbol). It’s easy enough to adjust the outer box of the symbol as you can apply parametric dimensions directly to the lines, but what about triangular filled region?

Start with you detail item family template, add the reference planes to control the overall dimensions of the symbol.

Then draw in the outer edge of the symbol, locking the lines to the reference planes

Next, draw in the filled region, making sure that the end point of each line snaps to an endpoint or an intersection of a reference plane. The locking of the end point to the reference plane is automatic.

When you’re finished, your symbol should look something like this

And when you change the Length and Width parameters, the filled regions will automatically adjust along with the rest of the symbol.

Make sure these parameters are instance parameters, load the symbol into your distribution board family and then link the instance parameters in the symbol to the related parameters within your distribution board family.

Now you have a symbol that scales correctly with the 3D components of your family.