It can be very small, it is also everything.

Luke 17:5-6 -  

The apostles said to the Lord, “Increase our faith!” And the Lord said, “If you had faith like a grain of mustard seed, you could say to this mulberry tree, ‘Be uprooted and planted in the sea,’ and it would obey you.

Bible Gateway link: Luke 17:5-6 (ESV)