matching suppy and return size

I have 1 unit apt. with forced air heat.Furance is in basement. Some rooms have 2 supply lines with floor vents and some have one. The return is one floor return in a room without supply line( kitchen). I would like to add cold air returns in as many rooms as possible. I have read using floor joists is not the right way to go because hard to get good seals.Should returns pipe sizing match supply size?? example. if room feed with 2- 6" duct it looks like 1-8" return would handle about the same volume of air. any thoughts?? sealed Joist?? round vs. Rectangular piping tin vs. flexible insulated tubing??