How do I change the default silkscreen font size in Kicad?
With KiCad (ver BZR 4022 Stable), it is easy to achieve this.

Open up your KiCad PCB and go to "Dimensions » Texts and Drawings".



On the Texts and Drawings window, you will see two textboxes that allow you to enter default horizontal and vertical sizes for text, as it can be seen in the below screenshot.

Enter your preferred values and click OK. 



Note: These textboxes may include values that has lots of digits after the decimal point. This is because of the KiCad's unit conversion system. If you switch to metric units, you will see these values are normal.