Gibbs, Jeffrey A. “Matthew 6:25-34: Food and Clothing." Matthew 1:1-11:1. St. Louis: Concordia Publishing House, 2006, pp. 359-366.
In Matthew 6:25, Jesus says not to worry about τῇ ψυχῇ. Gibbs finds this word in Matthew is not often used for the "soul" but much more often for the entirety of life (Gibbs 2006, 359). God is perfectly able to care for all the things we need, as evidenced by the οὐχί. The question expects a positive answer (Gibbs 2006, 360).
The amount of change we could expect as a result of worry, stated in 6:27, is slightly cryptic. Gibbs notes it could be taken as adding a small amount to the stature. However, a cubit is a large part of a human stature. The word can also be used for a duration of time, maybe a small amount of time required to move a short distance (Gibbs 2006, 360). Gibbs favors this view.
Gibbs observes that the teaching of 6:25-34 follows naturally from the teaching about treasure that preceded it. He also observes that Jesus is speaking about what is necessary to life, not what might be desired (Gibbs 2006, 362). The discipoles may find themselves in positions of dire need, such as the bird which starves or is captured by another animal, or the wild flowers in a time of drought. Yet Gibbs points out there is no need for eternal fear (Gibbs 2006, 363).
Rather than worry, Gibbs finds Jesus calling his disciples to seek God's reign and righteousness (6:33). The proposition is that in the end what we normally worry about is already cared for by God. Our concern is rightly focused on God's kingdom (Gibbs 2006, 365). Gibbs ties this command to the emphasis a Christian should have on receiving from Word and Sacrament, focusing on Jesus' work rather than our own troubles (Gibbs 2006, 366).