In the future, mention what version of Revit you do have access to...that would help...if I have that version installed. The orthogonal sketch segments are aligned and locked to the reference planes you can see in the images. The diagonal lines are inferring their relationship to the form from those orthogonal sketch segments.
General parametric Revit family advice: Build the bones first (reference planes), add muscle (dimensions/parameters) and then skin (forms). You could build the form at a specific size without any parametric ability with the same logic used here...without dimensions or parameters. But then you might have to rebuild it to deal with a different size later.

Steve Stafford
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