All of the recipes I've read seem to have small amounts of chocolate (7-12oz) at a time.  Will I mess it up if I make a large batch--48oz? Or should I break it down?

The only limit to making modelling chocolate is mixing the chocolate and syrup thoroughly.  If you can bake a batch of bread that makes 3 loaves, then you can certainly mix this amount of modelling chocolate in one batch. 

Thanks! I just wasn't sure if I was missing something regarding why all of the recipes I saw made very small amounts.