There are many wonderful books which include detailed information about sizing the parts of your figure so they all fit together. Any book by Susanna Oroyan is especially helpful with wonderful illustrations. Ditto books by Robert McKinley if you can find them as they are out of print. Once I have sculpted a head I've found the Wheel of Human Figure Proportion to be an invaluable tool for designing the rest of the character. Use it to design the head size in the first place say when you have a special accessory and want to correctly size your character to use it.
All the dimensions of your character can be learned from the size of the eyes you use. 8mm eyes measure about 5/16th of and inch across, use the measuring system you like best. I sculpt glass eyes into my polymer clay figures and often sculpt eyeballs from Paper clay, allowing them to dry then inserting into my sculpture in the same way as I do with Polymer clay. Measure across the eyes you plan to use, or go by the size they were sold as and make note of this eye width number to begin.
Here's how I go about mapping out a character.
Draw intersecting vertical and horizontal lines on a piece of paper. Use a ruler to make them cross at a true 90˚ The horizontal line is the "eye" line with adult eyes falling completely above the line and infant eyes completely below the line. I rarely sculpt children and never infants so seek additional advice if you want to design them.
There is the width of an eye between the eyes! Center one eye width along the center horizontal line splitting it on either side of the vertical center line. Mark these points which represent the inside corners of the eyes. Next mark another eye width on either side starting from the inside corner marks to find the outside corners of the eyes.
Measure down from the center horizontal line along the vertical line one eye width and mark this point which represents the base of the nose.
Measure down along the vertical line another eye width from the base of the nose and mark the point which represents the line between the lips.
Measure down the vertical line from the lips another eye width to mark the point of the chin.
You have now divided the lower half of the face into 3 equal parts.
3 x eye width = 1/2 head length. 6 x eye width = head length from top to tip of chin.
Now that you know the head length you can use it to calculate the size of your figure. The standard for adult figures is 7.5 heads tall, fashion figures are often 8 or more heads tall, my figures are often less than 7.5 heads tall. The Wheel of Human Figure Proportion works on the 7.5 heads principle.
On a large piece of paper draw and oval of the head length you have calculated. Make a vertical line and divide it into 7.5 head lengths with the oval for the head taking the top space and the feet below the ankle taking the 1/2 space.
Use the Wheel of Human Figure Proportion or other references to help you understand where the parts of the body fall onto this marked line. Complete your stick figure drawing making marks showing where to position important parts of your figure like the ankles, knees, wrists, elbows, shoulders etc. You can use this stick figure to help you plan an armature for your character. Note that shoulders are usually 2 head lengths wide, this will be very wide for a female figure.
To begin sculpting create and egg shape of clay a little smaller than your finished size to allow for clay additions. The narrow end for the chin and wider end the top of the head. Transfer the face map you created on paper onto your clay face marking the center vertical and horizontal lines, the corners of the eyes, base of the nose and lip line with the side of a pin. Try to preserve this map as long as you can while sculpting to help keep the features where you want them.
A Bit More
The ears generally fall between the base of the nose and the brow ridge. The front edge of the ear, where it blends into the cheek, is about half way, front to back, on the side of the head.
Hands are generally sized with the heel of the hand at the chin and finger tips close to the hair line. I keep a file of paper face maps and their hand patterns in various sizes.
You can use your paper face map to plan a cloth head too. Mark the same points to help with painting, needle sculpting or positioning the features.
I save the face and body map drawings in an envelope marked with the name of the character so I can make something similar without having to start from scratch. I often make lots of notes on my body map page about pose, textile ideas, the base etc and then add a finished picture of the piece to the envelope.
I really depend on my C-Thru ruler to help me with all the above steps. A 6 inch ruler with an 1/8th inch grid marked in red is my favorite. I can look through the ruler to position the eyes so they are tracking together, identify what's going wrong ( oh gee the left eye is an 1/8th inch higher than the right) and make all my face mapping marks on the clay head. I get these from www.misterart.com search C-thru rulers.