Want to increase MemoryStream size

string[] fileLists = Directory.GetFiles("C:\\Downloadable");
        string strNow = String.Format("{0:MMM-dd-yyyy_hh-mm-ss}", System.DateTime.Now);
        using (MemoryStream mstr = new MemoryStream())
        {
            using (ZipOutputStream ZOS = new ZipOutputStream(mstr))
            {
                for (int i = 0; i < fileLists.Length; i++)
                {
                    byte[] content = File.ReadAllBytes(fileLists[i]);
                    string fileName = fileLists[i].Substring(16);
                    ZOS.CompressionLevel = CompressionLevel.BestCompression;
                    ZOS.PutNextEntry(fileName);
                    ZOS.Write(content, 0, content.Length - 1);
                }
                byte[] zipFile = new byte[(int)mstr.Length];
                Response.Clear();
                Array.Copy(mstr.ToArray(), zipFile, (int)mstr.Length);
                Response.AppendHeader("content-disposition", "attachment;filename=" + strNow);
                Response.ContentType = "application/x-zip-compressed";
                Response.BinaryWrite(zipFile);
                Response.Expires = 0;
                Response.End();
            }
        }