I prefer a classical guitar, since it has a smaller body size than an accustic one.

If you want the metalic sound (or more 12 strings) you'll have to get an accustic one, though if you consider a Banjo a kind of a guitar you can always take that since it smaller (though not everyone likes the sound of it ;-) ).

There is another option, which is to get an electric guitar and get a battery powered small amplifier like this one.