Like single bikes, tandems can be made from lots of different materials. Aluminum, mild steel, CM steel, Titanium,carbon fiber, or a mix of any or all of the above.
As to sizing: of course it depends on the material, but in all cases the siz of the frame tubes, must be such that the frame is strong enough for the job at hand. Touring bikes need the strongest frames, while bikes designed for light weigh riders need not be as strong.
There are many books available for bicycle design, far fewer for tandem design, but in any case you need a lot of experience building single bikes, before starting a tandem.
Do some Google searches for bicycle design, and you'll find a lot of information