"The apostles said to the Lord, “Make our faith greater.” The Lord answered, “If you had faith as big as a mustard seed, you could say to this mulberry tree, ‘Pull yourself up by the roots and plant yourself in the sea!’ and it would obey you."

(Luke 17.5-6 GNB)

Jesus used mustard seed as an illustration, because it was / is commonly used in that region's cuisine. So the apostles knew what it looks like. According to Wikipedia, mustard "seeds are usually about 1 to 2 millimetres".

I presume the apostles wanted greater faith so they could do miracles like Jesus. And eventually they did.

But there is also another lesson.

"I don't have great faith. I have a great God!

When you have a great God, you don't need a lot of faith - just enough to take Him at His word. ... The size of someone's faith is not important. What is important is the will of God.

What the Lord is saying is that, if it's God's will (as the initiator) that you move the tree (Luke 17:6), all it takes for you to move it (as the responder) is the faith to speak the command.

The real question is always, 'What has God said?'"

Bob George, "Classic Christianity"

~

See: Size of miracle