Sounds like a lot of you guys would be push mowing every yard you had that has a fenced in yard!
If you only had one choice I'd have to say 36". You'd be screwed if you had real big open areas but you would still be able to cut them. It would take you much longer but how much time would you lose using a 21" for all of those properties that have gates?
I guess the question could be looked at 2 ways. Vote based on the customer accounts you have now. What about if the vote was based on not knowing who your customers were going to be? Like starting a new business.
A 36 obvioulsy isn't the most productive and doesn't leave the best looking stripes compared to a 52, but at least you can cut just about everywhere with it.