Should a Synth, for example, desire to have a different stature, be taller, wider and bigger, they can choose a blueprint with exactly these attributes. As soon as the Synth has replaced their old blueprint with this new one, the blueprint will detect that the body, in its current states, deviates from the plans stored within the blueprint. It then instructs nanites to re-structure the body until it is of a shape and size that fits exactly with the plans it has stored.
This not only affects body size and shape, but also the amount and types of inner organs a Synth has. Some frames, for example, require additional systems, such as hydraulics, to function properly. If a Synth does not yet have one or more organs laid out by its blueprint, its nanites will slowly assemble the organ and “grow” it within them until it is functional.
Blueprints are also responsible for providing nanites with instructions to heal wounds and to repair both internal and external parts. That means that cuts to the outer layer of the skin will be repaired, just as much as internal damage.
While blueprints can dictate what a Synth's insides will look like and what stature they will have, they don't dictate the makeup of the Synth's exterior. The blueprint can, for example, provide instructions on how to repair damaged skin, but won't dictate what that skin looks like. This allows Synths to choose the look and feel for their exterior much more flexibly and also allows the use of modular limbs.
Every Synth always has one active blueprint. Blueprints are exchangeable, allowing Synths to choose between different body shapes and appearances by simply choosing a different blueprint. These blueprints are available through a large online blueprint database that can be accessed by a Synth at any time. The chosen blueprint is then downloaded and installed.
Some changes in a Synth's body can take a long time, spanning from a few days to heal a wound, to half a year to change their stature. The bigger the desired change, the longer it takes for the change to happen. Synths can therefor not change body size from one day to the next.