Wraps come in different lengths and different methods of tying can be acheived with the different lengths. One often needs a longer wrap for a front carry than for a hip carry for example.
Most people find a 5 or 6 a useful starter size. As with ring slings it is often easier to start by using a striped wrap as then one can tighten stripe by stripe to perfect a tight hold. They also vary in fabric composition- a cotton wrap is a good starter but for a bigger child linen or hemp can make a sturdier carrier.
Wraps can be very expensive if one goes for limited edition handwovens but many companies make bargain wraps which are great as starter ones and cost under fifty pounds.