A mustard seed was a common example of something very small (see 13:19).
you can say to this mulberry tree, ‘Be uprooted': Black mulberry trees can live for hundreds of years and have wide and deep roots, making them very hard to pull out of the ground.
it will obey you: This does not mean people should use faith to perform random miracles. God has unlimited power, and those who trust him will see him act according to his will.