Practical question regarding ductwork

[beenthere]

Thanks again for the words of professional wisdom. I am wondering if I can step down to 24" round because that is the largest diameter available in flex pipe, everything else above is custom sizes. Also is there a direct equal relationship between sq. area of round duct and rectangular one?

I know that prices are not permitted here but can you at least give me an idea on the difference between all these materials, so I know where to spend most of my time.

24" will work. You still want it to make 2 90° bends. Velocity is up a bit, but should be ok.

Best to just call around and get the actual prices to know what the difference will be.