Screenly OSE + LED Screen 336x504 px without external scalar

Hello everybody,
I want to use Screenly OSE with a 504x336 resolution LED screen that is connected directly to a sender without a scalar proccesor.
I need to fit the image in there and start from the coordinate x = o, y = o.
Is there any way to do it?
I cannot change the screen size or the position of the screen.
Any tips for this?