Base size can be also affected by size of the wearee, wrap thickness, how much clothing is being worn, tightening preferences and wrapping qualities of the woven wrap. Wrapping a larger child, using a thicker wrap, wearing more clothes or a preference for longer tails may cause a size up.
If more people will be using one wrap, use the T-shirt size of larger person. Extra material can be looped around your waist.
There are a lot of types of carries, and for wrap of any length several carries could be used.