Small wheeled bikes (20") with a folding (or detatchable stem) and detatchable pedals can be quite small but it depends on the shape of your trunk.

Some peopel do not fold the folder but fold the stem and lower seatpost and store the bike under a bed or behind a sofa. Some fold it to a more square package and put it in a corner, in a cupboard, in a wardrobe or on a shelf.

More strong small wheelers:

https://vimeo.com/29645258

http://its-not-a-bicycle-its-a-demon...portfolio.html Scroll down.