getLocaStorageSize.js

function gen(n) {
    return new Array((n * 1024) + 1).join('a')
}

if (!localStorage.getItem('size')) {
    var i = 0;
    try {
        // Test up to 10 MB
        for (i = 0; i <= 10000; i += 250) {
            localStorage.setItem('test', gen(i));
        }
    } catch (e) {
        localStorage.removeItem('test');
        localStorage.setItem('size', i ? i - 250 : 0);
        
    }
}

console.log(localStorage.getItem('size'));