Bard - Cantrip - Message
Casting Time: 1 Action
Range: 120 Ft
Components: V, S, M (a short piece of copper wire)
Duration: 1 Round
You point your finger toward a creature within range and whisper a message. The target (and only the target) hears the message and can reply in a whisper that only you can hear.
You can cast this spell through solid objects if you are familiar with the target and know it is beyond the barrier. Magical silence. 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet o f wood blocks the spell. The spell doesn’t have to follow a straight line and can travel freely around corners or through openings.
My Comments: This seems to be a spell WotC decided to leave up to common sense. Sometimes they make sure to say things like "your message can only be 25 words" or some other limitation, and here they left that out.
So, as a DM, you will probably want to remind your players that this is just a cantrip and they can't recite Moby Dick and have the target respond with War & Peace. The duration is one round, so they should be limited to whatever they can say in about 6 seconds.