I can attest to the effectiveness of 21 inch commercial self propelled mowers. I use to mow only with these (actually am waiting on delivery of my first 48 inch rider). Most of the lawns I did were no more than 10k sq ft, all with gates and flower beds etc. Two guys running commercial 21's could zip in and out within half hour most times. Sometimes 20 minutes. A good chunk of my accounts were all within walking distance of each other. I planned routes accordingly and it was very profitable.
If you're going to be using 21's in any shape or form, they have to be commercial grade. When I first started I bought a yard man (was a nice model, so I thought) and had it trashed within 3 weeks. At the time I was hardly doing 25 lawns a week. I love my Snapper's now. They seen some abuse. Change the oil every 25 hours and keep'em clean, will last you a very long time.