Wurtz (Little Cado) is a dwarf varieties. The others will all require pruning, but the new movement in fruit tree growing is to keep trees at a height that allows you to pick fruit without a ladder. They are even doing that in some commercial orchards because it reduces liability insurance and they can get just as much yield per acre because they can plant the trees closer together.

Buy for flavor and ripening time, and just keep it pruned to the size you like. We have a 'Fuerte' for winter fruit and a 'Reed' for summer fruit. 'Reed' has the finest flavor of them all in many people's opinion.

To add to lgteacher's comment, keeping them small also reduces water needs. Avocado trees need significant water.

The UC Research Station in Irvine has been experimenting with Avocado pruning in order to find water saving methods for farmers; according to their research, stumping a mature large Avocado to 6' will not harm the tree and it will be fruiting again in 12-18 months.