Lets say I'm trying to model a vehicle, given that I have the length of it. If I enter the length using units, for example I input 4.5 meters into the y axis and start modelling with the blueprints of the car, what is the difference as compared to modelling it according to the blueprints and then scaling it up to size?
Thanks
The result should be the same if you scale your mesh in edit mode. However if you scale the whole object in object mode your object will behave differently when used with particles or in physics simulations. The particles and the forces on the object will be scaled as well. To avoid that problem you can apply the object-mode scale to the mesh by pressing [Ctrl+A] and selecting "Scale".
If you do not scale the blue prints to the real world size (example it says 4.95 meters long but in blender it is much smaller) then you will not be able to input any dimensions. You could model it all off of the blue print then just scale the model later, but while modeling any numbers become arbitrary.
There is no real difference. It just makes it easier to start in the units you're going to be using. This is more for making models you intend to print, or send down the pipeline to another stage of development. As long as you just remember to do it and doublecheck your measurements at the end and apply the scale to the object, it really doesn't matter.
That being said, there are several modifiers and textures that don't scale well. For example, a low resolution texture (not a procedural one) might end up looking fuzzy at a larger size. Aslo, if you have things like particle systems and complex parenting groups, it could mess the relations up, because it uses the math to calculate the relations based on the input. For example, 1000 hairs across one unit is quite a bit denser than 1000 hairs across ten units. Your model would become nine-tenths balder.
EDIT: To clarify, if you're only using hardsurface modeling on one object with simple materials, there is basically no difference at all.
