The mustard seed is very small. How can a faith that is very small do anything of significance? It has to do with the one whom the faith is placed. If your faith is placed in an all-powerful God, then yes, even a very small faith can do anything.
This very small faith, like the mustard seed, must be whole. A mustard seed that is incomplete cannot do anything. Just so, a very small faith can do nothing when doubt accompanies it. Indeed, how can you call it faith when doubt accompanies it? In such a case, the faith is incomplete.
Why doubt God? Has He failed us in the past? Perhaps, He has failed to give us what we want. But have we given Him faith that is whole? Indeed a faith that is whole, no matter how small, asks and receives. God, after all, only wants to give. But as a loving God and Father to all, He will not give us what is bad for us.
“God proves his love for us in that while we were still sinners Christ died for us.” - [Romans 5:8] God, our Heavenly Father, gave us the ultimate gift of His Son, so that we might be forgiven. He will surely not take away the very best from us, but give us the best, if we have a complete faith, even the size of a mustard seed.