Desks designed with wide open spaces underneath are ideal. Avoid drawers, and structures that restrict easy swiveling of the chair without bumping the legs.
Leg space is often associated with cramped airplanes or the back seat in cars, however sitting at your desk for long periods is just as much an issue.
We forget that the space underneath the desk (away from view) is important. Individuals like to clutter up this area. Avoid placing the computer CPU in this area and other items, including trash bins, spare set of shoes, purses, space heaters etc. The only thing in this area should be your legs and possibly a foot rest. If your legs are restricted then your posture will be forced to adapt due to movement limitations.