Ah, that makes sense…I totally misunderstood you there for a minute. That doesn’t necessarily mean that you will have to set max widths on elements for wider than 1366px or whatever you decide to start designing for. Making sure your app looks good at different widths and adding width settings to certain elements is always worth the extra work.
Take a look at some of your favorite sites at full screen 1920px, and you will probably notice the width of the page content has a max width near the 1300px range. If it looks like it is fully responsive without a content container, zoom out a little to make the page smaller and you may notice the fully responsive content ends up stretching really wide and doesn’t look as nice anymore. While the content in the container maintains a decent look when reduced to that size, really noticeable with full width rows on the page.
It might seem like its saves you some time designing for the widest screen size but it might not be necessary. Not to mention the amount of content you would need to completely fill all that space.
Just a thought, but you’re definitely heading in the right direction. I also think you are over thinking it a little, you can always change it as you figure out those sweet spots.
If it is your first app, try creating a cool landing page with a bunch of different elements (mostly to fill the page and figure out how some things work). Like a one page website that has some length to it. You can even setup your header with buttons/links that scroll the page to that section using the workflows. This will allow you to figure out exactly how to setup your responsive settings to get the right layout you are looking for.