I'm not sure which specific specs you are reading, but with Alexa you have the sensor size, the raw image size and the video size. All video outputs are 1920x1080. This is down sampled, not cropped, from the full sensor image. I think the raw file size is 2880 wide.
To answer your zoom question - that's a pretty extreme blow up. You will lose apparent resolution (sharpness) and the image will appear soft if you do this from the HD image. You will have better results if you use the raw image, but it will be best if you don't do such an extreme zoom. Or if you trick the shot by rotating the camera 90 degrees to get the benefit of the full width of the image. Recording ARRIRAW requires a special outboard recorder.