I use different methods for sizing and scaling depending on the needs of what is most efficient or accurate in the circumstances.
My most common techniques:
1) Establish the scale of the grid in the program. For example, in Wings3D one square in the grid of the viewport is one square meter. 3DS Max, Blender, or Deep Explorer have others. I have measuring tools which I have made in the past which I can quickly bring in to the viewport by importing or opening them in the program. The appropriate ones are chosen, for instance, 10 kilometer one for terrain levels or a height scale for characters ( These scales have height and length or height and width measured ). These tools are: image planes, images, tetrahedrons which have been subdivided, or custom made measuring jigs for unusual shapes or measurements.
2) Sometimes I import a previous made character or other model which is the appropriate or close to the desired size and shape of the new content to be used as a kind of template.
3) Parts of previous models may also be used or combined from different models to be used as a template. A case would be a ship which is the same class of build, such as aircraft carrier of a class (Iowa Class Battleships, or other class, are examples) I may use a whole model as a template for a future one or parts of it which are to be the same in the new model. Other times these parts my be transformed while keeping the original dimensions.
I always verify the model dimensions with a measuring stick or measuring square (similar to a carpenter's square) before declaring the model finished.
1) Generally build and measure the most important parts or art assets first.
2) Create the art assets and measure them which are most often viewed or used in the game.
3) Always prioritize a process of most important to least important and repeat the cycle until all assets are done:
Example of build, measure, and import in to game:
a) Terrain (Finish the land closest to the main character first if possible)
b) Main Character
c) Vehicle or building used by the main character
d) Weapons or tools used by the main character
e) Environmental assets used by or closest to the main character, such as rocks, trees, furniture, and so forth
Repeat the process with the second, supporting, or opposing character and all environmental assets like listed above here.
Note: In some games, the game engine might allow you to resize the art assets after import.
All assets should ideally be measured before you import into game to avoid distracting the artistic train of thought. Try to keep the technical things such as measuring the objects outside the game as much as possible to keep the in game art workflow as pure as possible. This results in most powerful artistic creativity.
Important: The more polygons and other aspects such as ray-tracing (should be avoided most of the time) in an area than the greater the hit on performance, so there are programs which measure these things - an important area of measuring art assets. For example, programs can measure numbers of polygons, vertices, edges, faces, surface area, and surface shader area within a given area or volume of the game scene. Some do this by core or threading, too. This is advanced but definitely crucial ways of measuring models or other art assets.