A 20-ounce model is usually considered to be the most versatile, heavy enough to drive large nails and yet limber enough to be able to extract bent and broken fasteners with relative ease. Because pulling nails puts a lot of strain on the handle, if you expect to need to extract a lot of nails, look for a hammer with a fiberglass or steel handle. Make sure it includes a comfortable vinyl or rubber grip that feels good in your hand.
If you don't anticipate the need to pull a lot of nails, a traditional adze-eye hammer with a hickory handle should be sufficient. The hickory will absorb a lot of vibration from driving nails, reducing the stress on the hand and wrist.
There are typically two types of heads, either a finish-head or a "waffle-head." A finish-head hammer is the most common, with a smooth, slightly rounded striking surface on the head of the hammer, whereas a waffle-head has cross-hatch grooves on the head. This type of hammer is usually much heavier and is used for driving a lot of nails in framing situations where the finished-look of the wood is not a concern. A waffle-head hammer will leave a very distinct mark where a finish-head will leave a flat indentation on the stock.