How to declare byte array dynamically in c# for large files

I am trying to convert a document in a SharePoint document library to byte array in restful WCF service. When I declared the byte array with the maximum size, i am getting the files size as the maximum that I had declared. I need to know how to declare the byte array dynamically.

Below is my code:

using (CSOM.ClientContext clientContext = new CSOM.ClientContext(SPserverUrl))
{
    DocumentID = "229"; 
    clientContext.Credentials = new System.Net.NetworkCredential(@"username", "pwd", "domain");
    CSOM.Web _Site = clientContext.Web;
    CSOM.List _List = _Site.Lists.GetByTitle("TestFiles");
    CSOM.ListItem listItem = _List.GetItemById(Convert.ToInt32(DocumentID));
    clientContext.Load(_List);
    clientContext.Load(listItem, i => i.File);
    clientContext.ExecuteQuery();           
    var fileRef = listItem.File.ServerRelativeUrl;
    var fileInfo = Microsoft.SharePoint.Client.File.OpenBinaryDirect(clientContext, fileRef);
    byte[] buffer = new byte[9469417];
    // how to declare the above byte array dynamically with the file size dynamically
    using (MemoryStream memoryStream = new MemoryStream())
    {

        int bytesRead;
        do
        {
            bytesRead = fileInfo.Stream.Read(buffer, 0, buffer.Length);
            memoryStream.Write(buffer, 0, bytesRead);
        } while (bytesRead != 0);

        string base64 = Convert.ToBase64String(buffer);

    }
}