Servos come in sizes from small and weak [< 20 oz-in] to larger and very powerful
[over 200 oz-in]. The so-called "standard" size is 44 oz-in, and these work well for
apps like pan'n'tilt, etc. They also work for many hexapod walkers, but are too weak
for most bipeds, which require a lot of torque for the leg joints.
Nowadays, many of the more powerful servos come in the same size package as the
standard servos, so can be used as drop-in replacements.
Servos come in analog and digital varieties, with the digital being faster and more
powerful, and usually a lot more expensive than the analog. Analogs will work for
most general apps.
For a standard servo, I usually use Hitec HS-322HD servos, as they have good gears and
will continue to operate ok with voltages down below 4.5V.