Your BASE SIZE is the wrap length that will allow you to do all of the full length carries. We determine it by which is the smallest size that you can use for a Front Wrap Cross Carry. No two mama bodies are alike, but in general:
most moms use a size 6 wrap for full length carries
parents under 130 lbs can often use a size 5 wrap for full length carries
parents over 180 lbs may need a size 7 wrap for full length carries (although there are many moms between 180 and 220 lbs who do fine with a size 6--more experienced wrappers can usually do more with a shorter wrap so a new wrapper may want to error on the longer side to be safe)
very petite moms can sometimes use a size 4 wrap for full length carries.
Size 2 wrap (rebozo): A Rebozo is very portable, folds up small in a diaperbag, can be tied around your waist, or used as a scarf. It's great for wet/dirty weather because you can put it on without dragging the ends in the mud. It's great for Summer because there is so little fabric around you and your baby. Because most of these shortest carries do not wrap entirely around the wearer's torso, the same carries can be done by people of very different sizes.
Rebozo carry -- front (tummy to tummy) -see video below
Saltwater or Freshwater finish to a base size carry
Tibetan Finish to a base size carry
A 4.1 or 4.2 meter woven wrap is often called a size 5. This is the SMALL full length wrap, the best size for petite mamas who are wanting to do the full range of wrapping. You don't have to be petite to enjoy this size. If you find yourself preferring shorter carries that are still to long to use a shorty, this may be perfect for you. If your tails are too long when you use a 4.6 meter wrap to do your favorite carries, you will probably be happy with a 4.1 meter length.
A 4.6 or 4.7 meter woven is known as a size 6. This is the MEDIUM full length wrap (also called Standard) and is the most popular woven wrap size. It is the size that fits most moms for doing most of the popular wrap carries. If you want a wrap that allows a petite mom and a larger pop to both have several options, this may be the perfect compromise that will neither drag too long for the mom, nor come up too short for the dad to have some options.
A 5.1 or 5.2 meter woven is called a size 7 wrap. This is the LARGE full length wrap. Plus size moms and dads will use this size for all of their full length wrap carries. It can also be used by someone who normally uses a size 6 but wants more room for tandem babywearing, or doing extra long carries.
A 5.7 meter woven is often called a size 8 woven wrap. This size is for full length carries for those who wear larger than a size 24 dress size (US) and should work well, for example, for a parent or caregiver who is in the range of 5'10 and 300 lbs. For reference, most popular brands of stretchy wrap are one-size-fits-all, and that size is around 5.5 meters. Just about a size 8.
A 6.0 meter wrap is available in some of the wrap brands, and is a great option for parents who find a size 8 to be not quite enough. You may need a larger wrap if you don't have enough tail to loosen your carry down for nursing, for example.