Currently, I'm designing a mobile app -using hybrid platform- for public use, and I'd like to know, which screen size I should use as my standard, so I can give the best user experience for my customer.
What's your product for? Do you have targeted an audience of users yet? Where do they live, what do they do, how are they interacting with your product?
I wouldn't necessarily decide on this upon "most used screen size".
I personally would think about the product, its placement on the market and my target audience first. Age, country, local internet speeds play a huge role when deciding upon designs especially on mobile.
The best experience does not come from a single most used device or screen size, but rather from an experience that works on any device you're targeting.
Well, you could see here the 2016 stats for all smart devices: https://deviceatlas.com/blog/most-used-smartphone-screen-resolutions-in-2016
You can see it also by country and choose the exact case you want.