Question
I want to establish a residential lawn trimming business...I need to be able to access back yards...which size riding lawn mower should I use.. which product is more durable and efficient?

Answer
I suggest a mower not more than 32 inches wide. If it is a walk-behind mower, watch the hand controls. You can bruise your knuckles quite easily (and painfully too!)against a gate-post or fence. I'd go for 7 gauge rather than 11 gauge metal for deck.
Those are my preferences - not industry or any other standards.