This is what the National Fire Protection Association (NFPA 720) says:
The location of the Carbon Monoxide detector for its effective performance is not generally dependent on mounting height. The density of carbon monoxide is similar to that of air at room temperature, and carbon monoxide generally mixes readily with air. The manufacturer’s installation instruction should also be followed.
Since there are usually several factors having impact on the air movement inside your house you may want to install more than one CO detector. The plug-in type at the electrical outlet level (16”) and another unit up on the ceiling so in case of a spillage you have everything covered. They aren’t that expensive and all recommendations (including the manufacturers) are usually the minimum you should do.