One of the questions that I get repeatable asked is which driver do I recommend and which motor. Unfortunately it's not a simple answer. You have to start by assessing how big of machine to relate to motor size. You can't expect an 80ozin motor to move a 50lb gantry reliably, or you wouldn't want to put a 800ozin nema 34 on a pcb router. What I can give you is some generalities to help you start to form a decision. To start with, there are small stepper motors designed for specific tasks, such as floppy disk drives and small inkjet printers. Don't expect a floppy disk motor to drive a cnc machine, and in all likelihood same goes for a small inkjet stepper motor. Standard Nema motor case sizes - Size 17, 23, 34, 42. NEMA (National Electrical Manufacturers Association) standards are for the motor case front view and the mounting flange holes. NEMA 17 (1.7” sq. or 40 mm) with a 5 mm shaft, NEMA 23 (2.3" dia. or 56mm) with a 1/4" shaft, NEMA 34 (3.4" dia. or 86 mm) with a 3/8" shaft, NEMA 42 (4.2" dia. or 107 mm) shaft size varies, frequently ½” or 5/8”. In perspective a strong nema 17 might drive a small pcb router with an efficient mechanical drive. Probably the most popular size is a nema 23. You can get a wide range of power from nema 23's, from less than 100oz in to over 400oz in. And finally nema 34's and nema 43's are generally used to retrofit commercial mills where you are moving large amounts of mass.
Here is a picture of a small motor that you might find in a desktop printer. This motor is rated at less than 10ozin.
Once you have decided on the size of the motor you need, when selecting motors look at the number of leads on the motor. 6 and 8 wire motors give you the flexability to use either unipolar or bipolar drivers. A 4 wire motor has to be driven by a bipolar driver, and a 5 wire motor has to be driven by a unipolar driver.
Other things to look for are motor coil voltage requirements and inductance. As coil voltage goes up, it become more difficult to get higher speeds for rapid movements, the same is true for coil inductance.
A further explanation of 6wiremotors.
A further explanation of 8wiremotors.
More information on stepping in various step modes.
Information on where to buy stepper motors at WhereToBuy#Stepper Motor Sources.
If you have question on the power of your motor and different ratings look at my Conversion page.