Selecting a bowl is a very personal thing. There are quite a few things to consider when making your choice such as size, weight, bias and grip. A key determinant that has is green speed where you will be playing. If you don't already know what this is, you can ask the coach at your local bowling club about this. Greens in the southern states tend to run slower than those in more northern parts of Australia. A faster green means that the bowl needs to be rolled to take a lot of grass (rolled toward the jack in a wide curve ) which makes the bowl take 14 - 19 seconds to reach the jack. Conversely on slower greens the bowl should be rolled to take much less green (in a slight curve), which means the bowls take less time to reach the jack say 10 - 14 seconds.
One way to get the right size, shape and weight is to wrap both of your hands around the widest running surface of a bowl so that your middle fingers touch at the bottom and if your thumbs touch at the top that is your size. If, when bowling, the bowl slips out of your hand or the bowl drops onto the grass then you should move down in size until you are comfortably in control of your bowls.