@abstract_hands0688 Saw your file. I couldn’t find a way to convert the rectangle to a non-bezier. If you’re worried about the animation, you can simply copy the Transform from your existing layer and paste it on the new layer.
If the Size property is missing, it may be because the square was created differently. I’d check the layer’s transform/path settings and make sure you’re working with a regular parametric shape rather than a converted path. That should make it easier to get the exact dimensions and set the extrusion depth correctly.
To make it a perfect cube, the Extrusion Depth should match your square's side length exactly. So if your square is 200×200 pixels, set the Extrusion Depth to 200. Same idea for any size — the depth just needs to equal one side.