i don’t know of this might help. I had also problems with fontsizes. I used Taifun’s Toolsextension.
I used this formula and it solved my problems on different phones.
In the designer the fontsize is 16 and looks likes this.
In my app it looks like
This looks the same on phones with a HD screen or on a Samsung Galaxy Grand Prime with a screensize of 540 x 960 pixels.