What would be the best screen size to develop a webapp with if I want to fit most of the phones on the market with ?
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The best screen size to develop for is all of them.
construct.net/en/tutorials/supporting-multiple-screen-17
I aim to have all aspect ratios between 4:3 and 16:9. Realistically speaking if your design can accommodate those ratios, it can probably already accommodate everything else too.