I towed a 6x12 single axle v-nose trailer from Michigan to Kansas behind a 2016 Suburban. The trailer was filled (completely) with everything my daughter needs for the next 4-5 years of school.
I averaged 10 mpg going, and 9 mpg coming back empty. I was expecting better mileage since it was single axle, and 6 foot wide.
Based on this, if I was getting my own a would get a tandem axle and go as big as I might ever need. I know that this trailer was purchased thinking that 6 ft wide and single axle would result in better mpg than anything bigger.