Most of the basics have been said by Richard. Only I would like to add that an oversized-head racquet can be a really dangerous choice.
The mechanics on these types of racquets are simple : most of the weight is concentrated on the head, so while they'll add significant power to your swings (because of the inertia created by your forearm speed), you will be wearing out your elbow. The reason being the enormous amount of vibration and power literally added by the head of the racquet to your swing.
You can see it this way : imagine a little kid, with a big baseball bat, if he can swing it, he will hit harder but not because of his own strength. Give him a little baseball bat and he will hit with less power.
In general, I'd say that your best choice is to go for small to mid-sized head racquets.
- Over-sized heads will add power by compromising control and feel;
- Small heads will give you more control and feel on your game, but they require more of your actual strength;
- Mid-sized heads can be a good compromise if you're just looking for fun and comfort.
The same principles apply for the beam width. Now, as a general principle, experience beats theory. So why not try the racquet itself to see how it feels.