That tank is the minimum I would recomend for a small uro...I admit it is too small for my 12" mali. It's 48"x13"x13"...roughly the same square footage as a 30 breeder, just longer and thinner....I would have preferred to have been able to find a 30 breeder because I like tanks that offer more depth, but I was really short on money and this was the only second hand tank I could find that would be big enough and fit my budget. Now the new tanks are coming along slowly, but nicely just the same, and they will offer a lot more room...4'x2'x2'...I'd say that's about the minimum for adult uros, though you can go with a little less height, especially if you don't have the heat lamps built inside the cage like I'm doing.
The dirt substrate you want to add some water to about 2 times a week...you'll start to be a better judge at just how much once you get used to it. Usually I lift the hide (I have it setup differently now, so they are stacked like a normal retes stack, so I could get more height and raise the temps up)...then I add water equally among the whole setup, just enough so that it's damp. Naturally, the parts not under the hides will evaporate and sink into the soil, leaving the top dry by the next day, but the dirt will still be moistened enough so that it provides a firm floor, and under the hides it will give the humidity that uros often seek out in burrows in the wild.
As for impaction, I used to keep my beardie on sand, and I kept my uro on it for a couple of days, and I noticed both of them would lick it constantly. Well, my beardie ended up still having parasites, so hoping to rid him of them once and for all, I put him on rubber shelfliner..though after brumation I'm going to put him on a thin layer of dirt aswell, and possibly try to grow some patches of grass and have a couple of potted plants...I've gotten side tracked, anyways, when I put the uro on the dirt, she licked it only once or twice, and immediately started perking up, and since then she's acted healthier than before, probably the healthiest she's been in her entire life. I haven't since seen her lick the dirt...I think she probably recognizes it at something that she shouldn't eat. There are small pebbles and stuff in it, and I've never seen her try to eat them. That's the only thing I'm worried about with my beardie, because he tends to try to eat things that can't be eaten, but I'll try it out and see, because it's a much better substrate for him too, being an animal that would naturally dwell on packed dirt in the wilderness, and not loose sand as many people think. I'm not saying loose sand is a bad substrate, some people use it with great success, but recently I've been realizing that it's probably a much better idea to try to give them the resources they had in the wild, because they'll know what to do with them more. Who knows, maybe some beardies do like to burrow, but most of us would never know because we haven't tried it yet.
So all in all, I'd say it's pretty safe. I don't see why a uro would try to ingest large amounts of dirt or any rocks. If you do happen to see it, or your uro or other animal stops eating and pooping for an extended period, check the temps, and think about taking it to the vet just as a precaution, but that goes for ANY other particulate substrate.
Hope I helped a little.