After swinging a hammer over a million times in my life I can tell you that it does matter on the size of the hammer. Here are some tips that can help you choose the right hammer for you.        

First look at the job you are going to do. Are the nails long and thick, as in framing nails. Or short and thin, as in finish nails. Once you decide this then figuring out the hammer is easy. You could use a big hammer for a small nail, but do you want to use a small hammer for a big nail? This is your option. Remember swinging a small hammer for large nails is going to require more and harder swings. Believe it or not these swings take a toll on a persons elbow and wrist, even if done properly.    

Once you decide on the size of the hammer head needed for the project, you need to look at the handle. Believe it or not this helps the arm with the right handle. A wise and experienced carpenter once told me that handles made of wood are easier on the body than those made of metal. "It absorbs the vibration", he told me. After a day of swing a hammer, and my elbow and wrist throbbing, I decided to change my hammer to a wooden handled one, and you know what...he was right. This is up to you to decide on, but I prefer the wooden handles.     

Then you need to decide on the length of the handle. Are you driving in big nails, then you should use a long handle. For smaller and shorter nails you may need a small handle. How thick should the handle be, well again that decides on the size of the nail. The bigger the nail the thicker the handle is always the rule. 

Last you need to decide if those nails you bend, you will bend some, need to be pulled out, or just left or pounded in bent. If you decide to pull the nails out the back of the hammer head should have a "claw", thus the "claw hammer". This will help you pull the bent nail out. If you decide to not pull the nail out then basically any hammer will do.