I am aware that when a noun followed by a number, the indefinite article is not used. For example: "Mike lives in room 65." But I have noticed that when talking about the size of something "a" is used. For example: "Can I have these boots in a size 6?" Why is "a" used there?
In the first example, the noun is a unique item; presumably there's only one Room 65 in Mike's building. In the second example, the noun is not unique; the shoe store may have multiple size 6 pairs of boots in that style. It's no different than asking "do you have this shirt in a [size] medium"?