Help with return air duct

I really need some serious help. I live in a townhouse. the basement ceiling is still opened, so I decided to inspected all the ducts that branches off the main supply. I realized that the contractor who build in the house did not install the duct properly. So right now, am going through some major duct work. I have a return air duct on the middle floor and in the hall way on the upper floor. My upper floor gets too cold during winter and too hot during summer. My current unit is 2tons and am in the process of upgrading it to 2.5tons or 3tons. Just to let y'all know, the living area of my town house in 1220sf. Not that big. Also, each room on the upper floor has two supplies going into them. That should be enough, rite, once I upgrade to a bigger unit? Okay, my major concern is the return air duct. I have on in the hall way upstairs. Can I use a 'Tee' duct on the existing return duct (located in the hallway), the run about 6ft duct work of return air to each room upstairs. I have access to my attic. So, if I split the return air to deliver to the two rooms and hallway, will that have an impact on the system. Also, how do I determine the size of the return duct for the rooms assuming this is possible.