I run an 8.5x20 it is a great size trailer. my father runs an 8x16 and it is just too small. After using the 8.5 wide i wouldnt buy an 8 wide. that extra 6" makes a big difference when moving around machines. another thing i would recommend is 5k axles. the weight adds up in these mowing rigs and they get a lot of miles.
here is a rough figure for my set up
8.5x20 - 3,200
56" ztr - 1,200
48" wb - 500
36" wb - 500
21" wb - 100
thats 5,500lbs in a 7,000 trailer you would have 1,500lbs left but by the time you add in all the weed wackers, blowers, racks, gas, misc. tools, oil, a barrel for grass, fill that barrel with grass, spare parts, ect. you will get that 1,500 no problem. now lets say your still good with your weight coming in at 6,500lbs and want to replace a walk behind with a stander or a ztr your now over weight.