I don't get much in the way of roots with most varieties while they're in the rainwater bucket, but I get a lot of leaf and branch growth (and even tiny brebas). Should note that I always remove any leaves from the cuttings before I use them.
I get the kind of roots in the photo Dieseler posted. But then when I transplant the leafy cuttings to their pots, the roots grow rapidly until I have no choice but to put them in the ground.
My success rate is 100%, but I usually dispose of the slowest growers when I realise I don't have enough pots.
My cuttings are always small, from 8 - 12 inches. Around 70 % submerged in the water, and I have to constantly top up the bucket come spring because I live in a hot climate.
I don't worry too much about the size of the roots. I pay more attention to the top growth instead, and transplant to soil when there are enough leaves and green wood. My roots are about the size of the ones in the photo when I transplant, but there are a lot more of them.
I live in Cyprus, which is a very humid climate, so I have no need to increase the humidity. Otherwise, I'd just use a simple plastic bag like I do when I plant veg seeds.
I think my warm humid climate has a lot to do with my success rate, but the most important factor is the rain water. Mains tap water with all its chemical additives won't do, but I have used well water to top up when I lack rainwater.
Some varieties take quicker than others, but I've found that if I stick with it, eventually all my cuttings have taken. The ones that took quickest always grow much faster when they're in the ground too, while the slowest one (always a late maturing variety) might only grow a few inches the first year.