To control the model of the rider in BikeCAD, go to the Rider Anatomy tab in the Rider dialog box. Editing any of these values will instantly be reflected in the model.
A simpler option is to move to the Fit advisor tab and enter values for the six basic body dimensions. Even simpler still, you can uncheck any of these boxes and let BikeCAD approximate values for them.
Ultimately, the changes you make here will not be reflected in the model until you click Calculate, then select the Rider anatomy tab on the right and click Apply to BikeCAD model.
How these sixteen rider anatomy dimensions are calculated is based on the customizable formulas that can be found by going to View >> Customize >> Fit advisor and then selecting the Fit advisor >> Rider anatomy tab. Here we see that each of the sixteen rider anatomy dimensions is some function of the six more basic body dimensions plus an optional constant.
How the unselected dimensions from the fit advisor are calculated is determined by the formulas in the Approximate missing dimensions tab.