The length/height of your grass plays a big part in how much effort is needed to do the job. Watch how the lawn grows along with your selection to help make your decision easier by giving you perspective.
Below the ankle would be your average lawn that gets cut regularly.
Above ankle are for those lawns that have been left a week or two longer than usual.
Shin to knee
are for lawns that can go a little crazy if left unattended for a while.
Overgrown
would be a bit of a suburban jungle and would need a mowbuddy to visit for a quote.