How do you choose what size phone to buy?

Let's get one thing straight: the hip pocket of average guys' jeans is deep enough to fit any size Android easily... in addition to a wallet,  a pack of gum, a pocket-knife, and an iPod; the hip pocket of average girls' jeans will grudgingly allow one end of an Android to be wedged in 2 1/4 inches, and will hold the phone in place only if the wearer stands still. In other words, don't base your decision about size on whether or not the phone will slip into your front pocket.

A reason to buy a big phone is that the more you can see at once on your screen, the more useful and simple the phone will be. A reason to buy a smaller phone is because it's more subtle and convenient. Personal preference determines where the balance  is between those two primary points.
Regarding thickness of the phone, there's really just one thing to say: thinner is better. Manufacturers like Samsung and HTC, particularly, are slimming down their phones beautifully. A feature that will add significantly to the thickness of your phone would be the inclusion of a slider QWERTY keyboard. This is just not necessary, in my opinion. Android touchscreen keyboards, Swype, and an abundance of additional options make a physical keyboard nothing but bulk.

A progression in phones recently has included the amount of phone surface that is actually screen, rather than frame. That means you can get a pretty good idea of the phone size by talking about the screen size. The information that smartphone screens range from about 3 to 5 inches, measured diagonally, tells you little. You need a frame of reference in order to determine how big a phone actually looks. A lot of manufacturer websites will show you the phone in comparison to familiar items, such as an army man or a credit card, but I've found that's not enough to get a good idea of its size. The easiest way to check out a phone's size without actually holding the device is by finding pictures of the phone in someone's hand. Look for reviews of the specific phone, for the most likely place in which to find these pictures.

UPCOMING POST: Final Installment of "Buying Guide" series -- Specific Recommendations