Using.cs

var response = await webDavClient.GetProcessedFile(uri, new GetFileParameters { CancellationToken = cancellationTokenSource.Token });

var size = response.ContentLength();

var destination = new MemoryStream();

byte[] buffer = new byte[bufferSize];
int read;
long totalRead = 0;
while ((read = await response.Stream.ReadAsync(buffer, 0, buffer.Length, cancellationTokenSource.Token)) != 0)
{
    await destination.WriteAsync(buffer, 0, read);
    totalRead += read;
    Progress = totalRead / (double)size;    
}

WebDavStreamResponseExtensions.cs

public static class WebDavStreamResponseExtensions
{
    public static long? ContentLength(this WebDavStreamResponse message)
    {
        FieldInfo fields = typeof(WebDavStreamResponse).GetFields(BindingFlags.NonPublic | BindingFlags.Instance)
            .FirstOrDefault(x => x.FieldType == typeof(HttpResponseMessage));

        HttpResponseMessage response = fields.GetValue(message) as HttpResponseMessage;

        return response?.Content?.Headers?.ContentLength ?? 0;
    }
}