Choosing a Lawn Mower

When visiting my uncle in Ohio I love to help him mow his lawn with those get on and drive lawn mowers. What seems to be a lot of work for him to mow the 5 acres of land on his property. Is just a lot of fun for me riding around cutting the grass and enjoying the landscape around me. Not all of us have 5 acres of land to mow. What if you have a small or medium size front or backyard. What size lawn mower should you use for that ?