Here’s a great tutorial on using Flexbox to make a (fake) messaging app – very interesting, as it’s done only for mobile:
http://www.creativebloq.com/netmag/create-slick-css-layouts-flexbox-41411325
If you’re having trouble predicting what your flexbox setting will do, here are two sites that let you doodle until you figure it out:
http://the-echoplex.net/flexyboxes/
http://bennettfeely.com/flexplorer/