Why is there a limit to level size

there’s no reason! why can’t I make big levels?

There are no scene size limits in the engine, other than what your system can handle via editing, or what the player’s system can handle when rendering.

If you’re talking about the scrollbars not going any further, that is normally limited by memory on your machine. However, you can just use the middle mouse button to click and drag to go past the limit of the bar.

I think you talk about the blue handles on side of the scene editor.

Press the spacebar and drag’n’drop with the cursor on the scene to move around.