There is a guide in the booklet you get in the starter pack called "Choose Success" that has the lowest acceptable target weight, its on page 25. For someone of 5ft5 the lowest is 8st 8lb and for 5ft 6 its 8st12, so the lowest you could set it would be 8st 10. On the next page, is a BMI style chart to show healthy ranges and the highest for your height is roughly (its quite small) about 10st 10lb or thereabouts. So setting your target somewhere between those two would be where you would be aiming for!
There is no official maximum level, but if you went under your acceptable minimum level by 4lb, you would have 4 weeks to get back to the minimum or your membership would be withdrawn. It doesnt say what happens if you go more than 4lb under.