You've raised a valuable question. Actually using of 再 in this way is quite common in Mandarin Chinese, but it has not yet been included in the major dictionaries of modern Chinese, or, accurately, has not yet been distinguished from a correlative sense: more. When you say 要是这双鞋子再大一点儿就好了, you may possiblely intend two different meanings: 1) you think the shoes are already big, or they have already been enlarged, but you want them ever bigger. In this case you put an accent on 再. 2) otherwise you think they are small, and you wish they were bigger or wish them to be enlarged. In this case you put an accent on 大 other than on 再. In both meanings 要是这这件衣服再小一点儿就好了 will make sense.

要是这这件衣服再小一点儿就好了。 << This sentence makes sense? Keen: Yes, it makes sense. In this expression, you use it to express your feeling of a little bit regret.

Sometimes the explanation in Chinese dictionaries for some words or some senses of them may be bewildering people who are learning Chinese.