The TrueTypeKey takes in the path to your ttf file relative to your assets directory, a Style and a point size. Additionally you can specify an outline size, for BMP text only, and the screen density in pixels per inch. When a screen density is specified the font will scale to match 1/72nd of an inch per point.
For bitmap texts it is often useful to scale the text down to create a nice crisp looking text. This is often not necessary on mobile devices as their dense screens tend to take care of this.
The above will provide a 21 point font, but all the text returned will be scaled down to 16 points. I recommend you experiement with different scales until you find a good quality for your font, however, a general rule I found that seems to work well is: