Cool, Kadri. But I really don't see any problem with making a scaled reference object directly in TG2.
Henry stated above, "I use the landmark object. It can be set to any size and placed anywhere.". Which is exactly right, and it won't appear in your render like a sphere/rock/disc/etc, as it is simply a preview-window reference point.
There are 8 different shapes to choose from for the preview display of a landmark object, I'm quite sure that a 2m(approx' 6ft) 'flag' or 'arrow' shape could be imagined as the simulacrum of a human being. Think ye not?
Maybe another 'figure' shape could be added to the list of landmark preview display options, I don't see any problem with using a default 'flag' landmark as a visual scaler, maybe I'm missing something here...