Here are some pointers that you should consider.
How strong are you? If you have good shoulders, biceps and strong forearms, you can go in for heavier bats. Heavy bats are good for big shots and they also last longer than light ones as they do not break easily. A good way to test if your bat is too heavy for you is to hold it with one hand at the top of the handle. Now raise your extended arm above your shoulder height and hold it for about 20 seconds. If you cannot hold it comfortably for even 20 seconds, it is too heavy for you. Remember that you will have to use it for at least an hour if you want to make a good amount of runs, the bats weight should be something that you can manage as you get tired.
How high is your back lift? If you have a back lift like the one Brian Lara had, you better stay away from the heavy bats even if you are strong. Big back lift players rely on bat speed for their shots and you will be best equipped with light or medium heavy bats. If you however have a low back lift like Sachin Tendulkar or Paul Collingwood, you can try heavier bats for more punch in your shots.
Long handle or short handle? Long handles are not very popular these days. If you are however quite tall and play your shots standing upright, a long handle bat is quite good as you will use the middle a lot more than with a short handled one.
Where do you prefer the sweet spot to be? Some players like it low, some like the sweet spot to be in the middle, some like it to be evenly spread. It is all a matter of personal choice. Once you have decided where your sweet spot should be, tap a cricket ball at the store to assure yourself that it is exactly where you want it to be.