Suggest finding used Co-Motion Periscope. Several models available; check out Co-Mo's website for more info.
The 'Scope is ultra size adaptable!
It features a seatpost-within-a-seatpost for ultimate height adjustment. Stem/bars can be adjusted by utilizing an adjustable pilot stem (Co-Motion sells those) and adjustable stoker stem (pretty well standard on many tandems).
Have a local couple that switch captaining positions. He is 6'2", she is about 5'2".
Not only are they able to fit as adults properly, she also rides as captain with each of her kids!
They did have to get the Periscope model with 26" wheels instead of 700c wheels to allow her proper standover room as captain.
Another possible choice could be a Bike Friday tandem. Features small 20" wheels and is quite adapatable but not as well so as the 'Scope in our opinion/experience.
Go for it!
Pedal on TWOgether!
Rudy and Kay/zonatandem