I still use the 6.5 x 12 single axle trailer I had built in 2002, when I started my business, for my mowing route. It fits a rider and a walk behind with room to spare. I added a 7 x 12 dump trailer in 2006.
I generally discourage guys from buying 4 x 8 and 5 x 10 trailers, but I think a 6.5 x 12 single axle is a great size for a small operation needing to haul a walk behind and a rider. An open trailer really isn't the best for hauling materials anyway...better off putting sideboards on the truck for mulch and brush and such and having the big/heavy loads delivered.