The way the map is scaled to the screen is controled by the
projectionMatrix passed into the
Take a look at the Tiled demo code to see how it works.
var viewMatrix = _camera.GetViewMatrix();
var projectionMatrix = Matrix.CreateOrthographicOffCenter(0, GraphicsDevice.Viewport.Width, GraphicsDevice.Viewport.Height, 0, 0f, -1f);
_mapRenderer.Draw(_map, ref viewMatrix, ref projectionMatrix, _customEffect);