Jesus said faith the size of what could tell a mountain to move?

Answer: A mustard seed

When the disciples couldn't drive out a demon, Jesus said faith like a mustard seed could move a mountain.

“Afterward the disciples came to Jesus privately and asked, “Why couldn’t we drive it out?” “Because you have so little faith,” He answered. “For truly I tell you, if you have faith the size of a mustard seed, you can say to this mountain, ‘Move from here to there,’ and it will move. Nothing will be impossible for you.””— Matthew 17:19-20 (BSB)