Odd-size RenderTexture for rectangular minimap

I'm making a minimap, which resizes depending on the size of the playing field in the scene. The normal size for at scene in my game is 100 x 100, but I want the minimap to support eg. 150 x 100.

An orthographic camera is generating a RenderTexture, which a simple NGUI Texture displays to the user. I'd like to change the size of the viewport/aspect ratio/size or the camera and the minimap texture, so the camera adjusts to the biggest size of the playing field. (Width/heigth)

How can I make the render texture be oddsize, and display more of the map, than just a square?

Cheers!