Ok, if you remove the handlebar it should fit into the trunk of your car unless you drive a compact car, if it doesn't fit, go with the 20" wheel. I used to ride on a 26" wheel and since i have discoverd the 20" i am not going back to a full size bike. Last week my wife and i were starting our ride with a huge smile since we noticed a young couple doing some bike wrestling action trying to fit 2 full size bikes to their mid-size car.... man it was fun, foolish guys, i felt sorry for them since i used to do the same thing a few years ago.

Anyhow, back to the 20" wheel, you just need to get used to it, put some miles on it and you will love it.
Yesterday my buddy rode my Dahon Mu P8 and was very impresses, he said: "WOW it rides like a regular bike..." and it's true, it feels like a full size bike.
If you go with the 20" wheel check the different models Dahon offers.
There are other brands like Downtube, Brompton, Bike Friday etc.. but i don't know much about them.