I'm planning on installing central heat and air in my home. My question is..
Where does the return need to be? And what size should the ducts be from the plenum to the rest of the house?

Most central air/heat systems are set up with the supply ducts and registers on the outside of the house, and the returns near the center. How big the ducts need to be depends on how big the house is, how many registers, and how far they need to run. You can pay a professional to calculate how big you need, or maybe just go on the large size and hope for the best. My 2 story, 2000 sq. ft house has about 16'' X 20'' ducts running from the plemum. You get the most efficient air flow if the ducts reduce in size at the branches.

Unless you are a licensed AC tech, you will need some help charging the AC. I think they banned the old precharged AC components in 1996. Might not hurt to have some help with the furnace. They can test the system and make adjustments to compensate for the duct sizes. I think the best place for the furnace and A coil is the center of the house. That can complicate the venting on a retrofit. Most furnaces and modern codes require a 4'' type B vent. You may need to locate the furnace near one end to simplify venting. If you go to an expensive, highly efficient condensing furnace, venting is much easier. Read the literature that comes with the furnace and follow it carefully.