Have you tried BytaFont 2? Simply put, it changes the entire font on your device. There are a ton of different fonts to choose from on Cydia.
Subtle lock uses whatever font Bytafont tells it to use. So if you want the text on your lockscreen to be bold, install that type of font. This is the only way I know that you can make the font bigger other than editing the tweak itself. The only downside is that it's going to change the font throughout the entire system.
The font I use is called Lato if you're curious.