Change The Size of the SubWindow and the Area It Covers In Unity

I'm creating a simple 2D racing game in Unity. The game has another subwindow that displays the enemy's view. Kinda like a small screen on top that lets you know where your enemy is or what he's doing.

Currently, I'm using a secondary camera to follow the enemy and a render texture to limit the display of the subwindow as well as its size.

However, I want the size of the window to be flexible like if i want the ratio of the window to be 4:3 instead of a perfect square. With my current implementation, whenever I rescale the subwindow, it just rescales everything up including the view being displayed. What I want to happen is, when I rescale the subwindow, the area being displayed should just be wider. It should just cover more area because I made the window wider. I wan the view to be independent.

Is there a way to do this with my current implementation? If not, how can I achieve what I want?

I'm new to Unity so I really hope someone could teach me. Thank you so much.