The sizing of your bets is easily one of the most important issues in poker and gambling . Being the betting game that it is, poker requires players to be downright adept at sizing their bets if they want to be successful. The incorrect sizing of the bets is a major plague for beginners. The reason is that everyone assumes that there is absolutely nothing wrong with his bet-sizing, as a matter-of-fact, it would be an insult to state otherwise. Unfortunately though most people do not understand why they need to size their bets the way they have to in order to turn a profit at the green felt.
Most online poker players have a whole lot of assistance available when it comes to sizing their bets. Online poker rooms are quick to provide their players with “bet pot” and “bet ½ pot” buttons that they can pretty much mesh without ever thinking about why they do what they do.

Players who win at poker never play just for the sake of playing. They always do things with a clear goal in mind, and therefore everything they do is done in the service of an ultimate goal. When they raise, they are either trying to make their opponents fold or they’re aiming to get money into the pot. This is exactly where you should start out as well. Know what you’re trying to accomplish in the hand, and size your bets accordingly. That is not to say that you should always bet the same amount in the same situation, because that will just make you utterly predictable, and in the end it will work against you. You will of course have to learn to mix up your play while in the same time keeping your bet sizes as close to ideal as possible.
Don't expect me to give you actual amounts when it comes to betting. Suppose you aim to make your opponent fold. The basic rule in this situation is to bet as little as possible while getting the job done. The reason is a simple one: some people will call you no matter what they're sitting on and no matter what sort of read they have on you. By betting less than a full sized pot, you'll be saving money when you do get called. Most people will fold to 1/2 to ¾ pot sized bet just as often as to a full pot-size one. Therefore, the benefits of just hitting that pot-size bet button are little, and the potential risks far outweigh these benefits.

By analogy, your goal when making a value bet (one that you hope your opponent will call with a hand worse than yours) is to make the highest possible bet he will be willing to match. What this translates to is that you're much better off making an oversized bet that your opponent will not call as often as he would a bunch of smaller bets.
In both situations, you need to have a thorough insight into your opponent’s playing style in order to become a winner poker player. Keep the above discussed two rules of thumb in sight and make your reads. At the end of the day, the reads that you make are more important than anything else, and sizing your bets is really a no-brainer when the above detailed two basic principles sink in and become second nature to you.