Aggregate size and mix ratio for concrete sculptures
There are two scenarios to consider:

Molding and Sculpting

For molding, most use a 1/4" to 3/8" aggregate.  Using larger aggregate may force aggregate to migrate to the surface.

The ratio depends on how big the statue will be.

Typical mix:
   1 part  Portland Cement (white cement ratio is the same)
   2 parts Sand (nothing fancy - its the connector for the cement to grab hold of)
   1 part  Aggregate (Gravel) - this can be as big or small depending on preference.
           most use the Pea Gravel sized rocks for statuary.

It may be necessary to increase the amount of aggregate to 2 parts in bigger pieces like the life-sized ones as described.

Additional things to add and think about:
  1) Add a Acrylic Bonding Liquid to help the mix stay together better.
  2) Add Alkaline Resistant (AR) Fibers and Mesh to the pieces to help with strength.
  3) Add metal wire - Rebar or stiff wire - to help with strength and flexibility.

Pointers for molding:

Concrete in molds can trap air bubbles and distort the surface of the piece.  To combat this, vibrate the mold to release air bubbles - tapping the sides of the mold or vibrating the entire mold with a mechanical device.

Experiment with making the mixture thicker or thinner to allow the bubbles to release without needing to vibrate them away.

If there are voids created by bubbles or mold issues, create a slurry of cement and water to fill the gaps and smooth them over

To finish the piece use a stone rasp, file, grinder, or sand paper while the surface is still damp.  Be careful as the surface is not cured and may mar or damage easily.

For actual sculpting:
   1 part  Portland Cement
   2 parts Sand

This mixture much less stable and can be shaped and worked.  This is really just a mortar and not concrete.

When making a large structure, something bigger is needed to support the cement.  Different ideas for use are: 
Styrofoam (Extrude Polystyrene - EPS), 
Backer board, 
wood,
metal, 
wire, 
plastic, 
paper, 
cans, 
glass bottles, 
and anything to take up space and provide support.  

Structures can be similar to the type used with topiary designs, but these won't be hollow.

After making the piece, it's important to let it cure and to seal it.  Any concrete exposed to the outdoor elements will deteriorate and lose it's details if left unprotected.  Clear varnish or sealers are the best choices, but be sure to use Outdoor products.

Curing times vary depending on the thickness, humidity, and temperature.  The longer, the better.  Keep the piece from undergoing extreme changes (temperature and humidity) to minimize the cracking and possible powdering/flaking. A slow cure with a stable environment is best.

The longer the cure, the stronger the piece will be.  For smaller pieces, wrapping in plastic is the most common way.  Misting or wetting the surface will help the humidity to remain stable and promote the curing.  The curing can take from 3-4 days to two weeks.  The process is done from the outside inward.  Many cracked concrete slabs that were poured years ago and are still damp in the center.

Some of the better instructional videos can outline specific methods and procedures.  

There are good YouTube videos about concrete casting and sculpting from these channels:

Rubbermoldman (aka P. David Jensen)
Mrconcreteartisan
Microcast Molds
T.TB- Construction - Specific to sculpting with mortar on top of other pieces.

Happy Casting !