Labels in Autodesk Civil 3D follow very simple yet useful rules.
In Model Space the label size is a multiple of the drawing scale and the label style text height. A label style text height of .1 in a 1:20 scale drawing would create a 2 foot tall label. A label style text height of .2 in a 1:40 scale drawing would create an 8 foot tall label.
In Paper Space it is even simpler. A label always appears the exact height of the label style's text height. This is irrespective of the viewport's zoom magnification also referred to as viewport scale. A label text height of .1 will always plot as .1 inches.
These rules also apply to labels that you xref. As mentioned in an earlier post that means labels are only created once and can be used for drawing at different scales.