Find Steed telepathy question

[Mud Puppy]

In session tonight our party descended into a tunnel 10ft underground and followed it. Our Paladin's Celestial steed couldn't fit into the tunnel with us, and rather than dismiss his steed he questioned whether or not the horse could walk along the surface paralleling the tunnel and maintain his telepathic link with the steed.

What say you playground? How much separation between Paly and Steed before the telepathic link breaks?

He ended up dismissing the horse before we descended but it was an interesting question.

[P. G. Macer]

Per page 240 of the PHB, “While your steed is within 1 mile of you, you can communicate with it telepathically.

[Mud Puppy]

That's what we get for none of us checking the PHB despite having no less than three copies at the table.... We just looked at the spell card.