Matt,
I have only had time to play a wee bit with the Button Builder, but it seems pretty straight-forward. Practise on the default button that comes with the programme, leave all the settings as they are.
Then set a breakpoint, say, around 700px, go to the fonts tab and reduce the size from 4em to 3 (or thereabouts) for the main text. You can reduce the font size of the secondary line too.
Then set a new breakpoint at appr. 500px, repeat what I said about reducing the font size. And so on and so on...
If, when you have reduced the text so that the secondary text can't be read any more, you can choose to hide it (on the general tab) and adjust the bottom padding for the button so that the main text is nicely positioned.
It's all about setting breakponts and playing around with font size, and possibly also padding.