Set max width, increase font size for article threads

[XFdestek]

Hi,

I don't have a sidebar, so my article threads span the full width of the screen.

What can I place in the extra template file to make the max width of the article thread content be 600px?

Would also like to increase the font size, but wasn't sure which option in style properties would do that.

Goal is to make the reading experience a little better.

Thanks for the tips!