You did not which watch model you own, but on the models supporting swimming, you can usually indeed set the pool size to any length 10m or more. Check out the settings of the Pool Swim activity.
I am not familiar with Lily, but on my watch I have also the option Custom at the bottom of the list. If you do not have it, then check out your user manual or post on the forum for Lily - other owners may be able to help better. If there is no way, then you'd need to edit the pool size subsequently in Garmin Connect.
