It really depends on your ultimate goal. Are you planning on creating a web app and wrapping it to launch in the App Store?
If it is just going to be an online web app without submitting it to a store than it might be helpful to create a fully responsive web app from the start.
You can check out some of my free videos on responsiveness if you would like to help get you started. https://www.nocodeminute.com/1-minute
It saves a lot of time in the long run when having one page and making it responsive, instead of having multiple pages and having to update both of them each time there is a change.
I always start my pages at 1200 pixels and then split it into sections.
1 Sections = 1200 pixels
2 Sections = 600 pixels
3 Sections = 400 pixels
4 Sections = 300 pixels
Then when you use the responsive engine it will be easier to make them into a mobile friendly version.
Just put everything into separate groups to make the sections you need.
Hope that helps!