I use a 19L bag (the mission workshop transit laptop bag) and it's perfect for a laptop, iPad mini, pens, notebook, travel mug, and a few other things. You'll probably be able to get everything you need in the mini metro since its 20.5L.
Another option is the
Mission Workshop Monty. Roughly the same size, only a little more expensive. I personally like MW bags more than Chrome bags. I own a bunch of Chrome clothing, but their bags always looked a little cheesy to me. I prefer the shape of the MW bags and they don't use the seat belt buckle which to me is a huge plus.