Edit in JSFiddle

<div id="app">
    <div id="tag-cloud">
        <span v-repeat="tag :tags" class="tag" style="font-size:{{tag.fontSize}}px">{{tag.name}}</span>
    </div>
</div>

#tag-cloud {
    padding: 3px 3px;
    width: 300px;
    height: auto;
    text-align: center;
    background-color: gainsboro;
}

#tag-cloud span {
    padding: 0 3px;
    text-align: center;
    display: inline-block;
}

var values = [    
    { name: 'January', count: 6, fontSize: 0 },
    { name: 'February', count: 2, fontSize: 0 },
    { name: 'March', count: 5, fontSize: 0 },
    { name: 'April', count: 1, fontSize: 0 },
    { name: 'May', count: 4, fontSize: 0 },
    { name: 'June', count: 3, fontSize: 0 },
    { name: 'August', count: 6, fontSize: 0 },
    { name: 'September', count: 3, fontSize: 0 },
    { name: 'October', count: 3, fontSize: 0 },
    { name: 'November', count: 1, fontSize: 0 },
    { name: 'December', count: 7, fontSize: 0 }
];

new Vue({
    el: '#app',
    data: {
        tags: values
    },
    created: function(){
        var maxSize = 28;
        var minSize = 8;
    
        // tags.countを降順にソート
        var copyTags = values;
        copyTags.sort(function (a, b) { 
            return b.count - a.count;
        });
    
        // タグの表示数
        var limits = copyTags.length < 5 ? copyTags.length : 5;
        
        var max = copyTags[0].count;
        var min = copyTags[limits - 1].count;
        
        // フォントサイズを決定し、表示する分だけthis.tagsに格納
        this.tags = [];
        for (var i = 0; i < limits; i++) {
            var perc = (max === min) ? 1 : (copyTags[i].count - min) / (max - min);
            var size = Math.round((maxSize - minSize) * perc + minSize);
            
            this.tags[i] = copyTags[i];
            this.tags[i].fontSize = size;
        }
        
        // tags.nameを昇順にソート
        this.tags.sort(function (a, b) {
            return (a.name > b.name) - (a.name < b.name);
        });
    }
});