By one device, do you mean the app is absolutely ONLY running on this device in a controlled environment? If it's not, then it is not reasonable to only design specifically based on that device.
By 1x, what I meant is to design the app at the base resolution (the default is 360x640). Since you didn't mention about which device is that, so I didn't know the pixel density, but let's assume that it is a Pixel XL/Nexus 6P with 3.5x pixel density, you can design the app at 412x732 (suggested by Marc) which you should be able to accurately preview your design on the device using the 1x assets from Material team.
But as I mentioned, if the app is not specifically running on that device itself only, then it's best to design the app at 360x640 to ensure the best layout compatibility with many devices out there.
Sorry, I wasn't clear enough. The app is in fact absolutely running ONLY on this device. :) Perhaps in the future it will run on some other similar device, but right now I'd like to have my Sketch mockups easily viewable on that exact device.The device in question is Lenovo Phab 2 Pro.
By one device, do you mean the app is absolutely ONLY running on this device in a controlled environment? If it's not, then it is not reasonable to only design specifically based on that device.
By 1x, what I meant is to design the app at the base resolution (the default is 360x640). Since you didn't mention about which device is that, so I didn't know the pixel density, but let's assume that it is a Pixel XL/Nexus 6P with 3.5x pixel density, you can design the app at 412x732 (suggested by Marc) which you should be able to accurately preview your design on the device using the 1x assets from Material team.
But as I mentioned, if the app is not specifically running on that device itself only, then it's best to design the app at 360x640 to ensure the best layout compatibility with many devices out there.
If you have some confusion for pixel density, resolution etc, this is a great read: http://sebastien-gabriel.com/designers-guide-to-dpi/