When I got my current rib it was at on a set of 8" "dinghy" wheels. Probably fine for the 200 yds it used to do per year at about 20 mph, but when I checked out, to get an 8" tyre rated to carry 3/4 ton per axle was about £100 / tyre!
I have 10" wheels on my trailer, primarily to reduce the likliehood of nav light removal on entry to the garage (and it makes launching soo much easier - I canlt remember the last time I had to unhitch to get the boat on or off). Got the higher rated tyres. More expensive to buy, but in the long run probably cheaper!