Lathes come in three different sizes. Small benchtop lathes excel at detail work below 1ft in length. Medium lathes are commonly used to turn spindles for chairs or railings. They operate best with workpieces of ~3ft in length. Large lathes go beyond the 3ft limit. The machines are industrial in nature.
A rotating headstock supports outboard turning. The head and motor turn away from the body of the lathe to allow the work piece to turn free of any obstructions.
Index stops lock the spindle in place. They are primarily used to hold the workpiece steady while carving. Adding flutes to table legs is an example.
The ability to change speeds is more important on a lathe than it is on most ofther woodworking machines. Lathes can spin a lot of mass. The rotation speed must be set lower for large work pieces. As the work piece gets smaller as it is being worked on, the lathe's rpms can can be increased to speed up the work.
Most lathes use a fixed speed motor in combination with a pulley system to create different spindle speeds. The user has to move the drive belt from one pulley to another to change the speed of the lathe. Models with a variable speed motor can change speeds at the touch of a button.
1) Choose a size
The swing and distance between centers limits the size of the workpiece that can be mounted on the lathe. Choose model that supports the work you want to do.
2) Choose a motor size
Next pick a motor that can rotate the workpiece at the speed your require. It takes very little power to rotate 2" chair spindle. A block of wood for a 12" bowl is a different matter.
3) Match the mass of the lathe to your work
Lathes are inheriently unstable due to the rotating mass that is the workpiece. If the workpiece is unbalanced, it will cause the lathe itself to vibrate. The weight of the lathe will dampen the vibration. If the lathe is not heavy eough and the unbalance severe, the lathe will start to walk across the room and posssibly tip. For large turnings, large lathes are a necessity.
Smaller, benchtop lathes have a maxium workpiece size. If you exceed this size, the operation of the lathe becomes a safety risk.
Smaller lathes can be made more stable by bolting them to the floor or by adding dead weight (sandbags, lead, ...).
For bowl turning a lathe that supports outboard turning is recommended. This feature allows turning without being encumbered by the frame of the lathe.