MemoryStream expandability

Hi all, I'm playing with using a MemoryStream as a buffer to build a document, then when I'm done I use the WriteTo method to dump to a FileStream. As I interpret the docs, the MemoryStream is supposed to expand as you add to it (using default constructor), but I am finding it caps at 1027 bytes. As I don't know the size of the document to start, I don't want to specify a size initially (which disallows growth past that size). Any one have words of wisdom on this?