Unit of measure for ET tube

The box where you can type in the ET Tube (endotracheal tube) size in the anesthesia preoperative section does not have anywhere you can enter the units. For example, we have sizes of ET tubes measured in mmid and in Fr. We thought we could just write in the units next to the number (for example, 57 Fr), but when you save it then get back in to the preop page, it is erased.
The ET tube size is just a number, but it doesn't mean anything because there are no units assigned.