According to https://developer.garmin.com/connect-iq/api-docs/Toybox/ActivityRecording/Session.html#createField-instance_function :units has at least 16 bytes. However it's misleading, because you can actually only use 15 bytes (i.e 15 ASCII characters) long text, because of the terminating null. There's no mention of null in :units, only in :count.
I'll add that the sentence "The maximum length may vary between products" is not very useful IMHO, as there's no way to know it for every device. I mean maybe one could test it on all the devices they want to support, and use the minimum of all of them. Not sure if we can count on the values in the simulator being true. fr965 is one of the latest products and it only supports 16 bytes (at least in the sim)
