Wow...can't imagine mowing lawns that size with a 21 or even a 36 and a 48 on lawns that size isn't a squeeze in my opinion. I have a lawn that's about 1,000 square feet that I mow with my 48. I run a 48 inch walk behind and a 52 inch rider. To each his/her own I guess.
Wait...you're mowing 1 acre lawns with a 21 inch mower for $25?