When modeling, it is important to create your 3D models to real-world scale.
Every 3D modeling application has some kind of units system, and every object in the scene is a certain number of units tall or wide. For example, a building might be 100 units high, which could internally mean 100 feet, 100 meters, or even 100 cm, depending on how you set up your scene.
Why Fix it?
You could take the attitude that the customer can fix your model's scale if he/she needs to. However, modeling to real-world scale will provide maximum opportunity for sales of your model.
The majority of our customers, by survey, want a 3D model that they can use immediately in their workflow. When comparing two comparable models for purchase, a customer is more likely to purchase the one that's already at real-world scale because it will work in any workflow, now and in the future.
Why Real-World Scale is Necessary
A model that isn't made to real-world scale will present a variety of problems to another artist merging it into a scene.
An artist merging your model into a scene will have to figure out the real-world size, either by calculation (time-consuming) or guessing (faster, but inaccurate).
Not all models respond well to scaling after they're built. For example, a rigged character or piece of machinery is nearly impossible to scale correctly. A model that includes a linked hierarchy can behave very strangely during animation if it's been scaled.
A customer who buys your model doesn't want to have to deal with scaling, especially when it's harder to correct the problem than to simply make it correctly in the first place. If a customer is choosing between two models, and one is made to real-world scale and the other is not, the model in real-world scale will win out every time.
Creating Models to Scale
Making models to real-world scale is a matter of setting a unit scale before you begin, and creating all objects in the scene to conform to this scale. Common unit scales are 1 unit = 1in and 1 unit = 1cm or 1m. Metric units are safest, as they are used worldwide and transfer well to other applications. See the videos below for information on how to determine whether your model is to real-world scale in your 3D application, and how to change it if necessary.