Have you ever asked God for greater faith? Have you ever desired increase in your faith? Maybe you are too polite to ask, but the disciples (who were around Jesus all the time) didn't hesitate. "Make our faith greater" they told Him boldly (Luke 17:5). Well, at least they adressed the right person. Jesus, however, holds them off a little and tells them to use what they have, however small that may be.
Nothing will be impossible for you (Matthew 17:20)
Oops, quite a bold statement, Jesus always knew how to trigger their thoughts. He basically explained what faith the size of a mustardseed is: something small that enables you to do big things!