How to measure the font size
In most common graphic editors the fonts are defined in points, which is a legacy measuring system from Germany. Fortunately it is very simple to convert points to pixels is very simple.
Determining the font size in existing application
The size of the font is defined as distance from the bottom of a descender (e.g. 'g' or 'p') to the top of a capital letter (e.g. 'T' or 'B'). The result is size in points which is equal to size in dp.