I used Casters Direct a few times, bought some 50mm ones for £10 a pop (2 braked, 2 not) for my router table and *something else*. But at £40 a cabinet they're too expensive. I tried a set of 4 from Amazon for £18 delivered, 4 braked and 80mm wheels, same blue rubber and width. They're miles miles better than the ones I bought from Casters, before comparing prices. I assumed they were a pricing error, but I was able to order another 4 sets, can't remember where from now though.
I've used some of the smaller lighter ones, but they couldn't cope with the weight of my planer/thicknesser and one of the rubbers ripped off the wheel, those were 50mm ones rated for the weight, but failed due to side loading when turning the caster on a vinyl floor.
You definitely need large diameter for a rough concrete floor and heavy equipment.
