When scaling artboards "by hand" with the "scale handle", the font-size of textframes changes as expected. However if I use the transform dialogue to enter an exact value I want the artboard to scale to, the font-size does not increase but only the area of the text-frame. It is the same as I would use the "normal" handles if I were to scale "by hand" instead of the "scale handle" (for the lack of better words).
Scale manually. Have the content grouped and use the handle which scales the content, too. Have another artboard which has exact dimensions and snap to it.