whoa the theremin is a superb instrument, I think the most aethereal ever created, not to forget that it is THE instrument that started it all! Actually is thanks to the theremin that we have today all of this electronic and digital synths, when it was invented in 1921 it was the first electronic musical instrument, indeed.
Btw, if you really need it because you think you're the next Clara Rockmore, then go buy an Etherware Theremin, it sounds perfectly! Or, if you're into radioshack and diy things, yuo might want to take a look at this:
following these instructions, you could buy your fit-in-a-pocket theremin or even your professional one.Just read this page :)