Snow White is in possession of the wicked queen's master magic mirror, which if activated in such a way that the user wishes, can create a link to pretty much any other mirror, and in this case Snow White can be choosy about who can use the mirrors (only those who would be allies and willing to stand against evil). Small hand mirrors would be able to be used for communication, but a larger mirror (even if only big enough to crawl through) can be used for bodily transport. But I suppose you could use a small hand mirror to send through a mouse or bird carrying a message or small weapon if necessary.