As you only need basic functions,any of the inexpensive ones would do the job.
The midrange ones have more features,but even then the quality and key action is nothing special.
As rictheobscene says,the AKAI LPK 25 looks slim.
I have a Novation Impulse,and they're kinda chunky.
Here's a link to the Thomann selection (up to 25 key).
Some have full size keys,some have wider end panels,so you'll get an idea of how bulky they are from the pictures:
http://www.thomann.de/ie/master_keyb...edcdb6880d126f