A good question. The short answer is that the number is a percentage of the overall screen height. So 10 is 10% of the height - for a 1024x768 screen the font height would be approximately 76 pixels.
A key feature in ZionWorx 2014 was maintaining the font appearance when moving between different environments (i.e. setting up at home and bringing to church) and we thought for a long time about the best way of approaching this and percentage seemed to us to be the simplest way of achieving this.