Figuring out proper bike sizing is not that hard, but it is amazing how often it is overlooked. This is especially true when parents are buying a bike for a child. Tempting though it may be, buying a bike that your child will "grow into" is just exactly the wrong way to select one. If the bicycle is too big and unwieldy, your child would probably prefer to walk. If they do try to ride a bike that is too big, that opens the door for some nasty accidents. Either way, they will become discouraged from riding. The same can be said when it comes to purchasing adult bikes. A good fit is extremely important, especially if you haven't been up on two wheels in a while.
For adult bicycles, the process is pretty much the same. Figuring out proper bike sizing means you allow between one and three inches between your crotch and the top tube. This should ensure a good fit. Again, with women's bicycles you have to imagine the location of the top tube.
As far as handlebars are concerned, they shouldn't be too close or too far away, and unfortunately, the only way to be sure about this factor is to keep adjusting them until you feel not only comfortable, but in complete control of the bicycle, as well.
After you have all of these details figured out, it's on to more pressing details, like Whats the best bicycle bell?