Currently there are two ways to set the height of a MaximizedItem - to set height to your whole RadTileView (as you've found out) or manually set height to the container of the MaximizedItem like this:
(container.TileState == TileViewItemState.Maximized)
container.Height = 500;
I've attached a sample project showing this so could you please examine it and if you have further questions feel free to ask.
All the best,
the Telerik team