Hi mlerf14,
Welcome to the HVAC Group!
When you get to Unit 70 you will find a lot of information on heat load calculations and how to properly size any duct system. It is very important to size a systems ductwork correctly because it can affect the operation and efficiency of it. If the ductwork is too small, it can be noisy and makes the air conditioner work hard since it is pushing air through too small an area.
If the ductwork is too large, it will not be able to maintain enough force to push it correctly through the building or house. The static pressure of a duct system is very important. Air flows through a duct because of a difference in pressure existing at one point and that at another point or portion of the duct. Static pressure is a measurement of the resistance that a duct systen presents to the flow of air. Just as the given value of static pressure will push the liquid in the manometer, it has the ability to push air through a duct.
Ducts have friction loss depending on the walls, material, elbows, turns, cross sectional area, transitions, etc. The larger the area of the duct, the lower the velocity of the air. Since velocity pressure depends on the velocity of air, a decrease in velocity air, because of an enlargement of the duct, will cause a decrease in velocity pressure. You may want to also watch some you tube videos on hvac, I find that to be very useful
Good Luck with your studies!