I am sure to start a hot debate here with this. Faith is not something that go's from a weak, to intermediate, to strong faith. There is no small, medium, jumbo size faith. Faith grows but how we think it does. "Faith does not grow in quality, but in quantity." A man of "great" faith is the man who has faith in all ares of his life. He believes in his salvation, he believes God provides all his needs for godliness, and life. There is no place where he is not having faith in. The man with "little" faith is the man who has faith in his salvation, but has no faith for God to provide the things he needs to carry his mission on earth or housing, clothing, food, ect....
Even though there is only "one faith" but this one faith can do multiple things all at the same time. Jesus said....
Mat 6:30 But if God so clothes the grass of the field, which today is alive and tomorrow is thrown into the oven, will he not much more clothe you, O you of little faith?
Mat 6:31 Therefore do not be anxious, saying, 'What shall we eat?' or 'What shall we drink?' or 'What shall we wear?'
Jesus was not telling them there faith was small, only they had little faith, which means they needed to have faith in all of their necessities of life, not just in a few things. When a person has faith it is already at it fullness, it can not get any stronger that what it is now. You either have faith or you don't. You can even have faith and unbelief at the same time.
Mar 9:20 And they brought the boy to him. And when the spirit saw him, immediately it convulsed the boy, and he fell on the ground and rolled about, foaming at the mouth.
Mar 9:21 And Jesus asked his father, "How long has this been happening to him?" And he said, "From childhood.
Mar 9:22 And it has often cast him into fire and into water, to destroy him. But if you can do anything, have compassion on us and help us."
Mar 9:23 And Jesus said to him, "'If you can'! All things are possible for one who believes."
Mar 9:24 Immediately the father of the child cried out and said, "I believe; help my unbelief!"
This guy had faith in believing Jesus was the son of David the messiah, but he had no faith in believing for the healing for his son. If he knew the same faith he had for believing Jesus as the messiah, could also be used to heal his son, but he did not know that.