(As of 2018, I would advise trying out JupyterHub/JupyterLab. It uses the full width of the monitor. If this is not an option, maybe since you are using one of the cloud-based Jupyter-as-a-service providers, keep reading)
(Stylish is accused of stealing user data, I have moved on to using Stylus plugin instead)
I recommend using Stylish Browser Plugin. This way you can override css for all notebooks, without adding any code to notebooks.
We don't like to change configuration in .ipython/profile_default, since we are running a shared Jupyter server for the whole team and width is a user preference.
I made a style specifically for vertically-oriented high-res screens, that makes cells wider and adds a bit of empty-space in the bottom, so you can position the last cell in the centre of the screen.
https://userstyles.org/styles/131230/jupyter-wide
You can, of course, modify my css to your liking, if you have a different layout, or you don't want extra empty-space in the end.
Last but not least, Stylish is a great tool to have in your toolset, since you can easily customise other sites/tools to your liking (e.g. Jira, Podio, Slack, etc.)
@media (min-width: 1140px) {
.container {
width: 1130px;
}
}
.end_space {
height: 800px;
}