The same approach of optimal range of sizes can be applied to Lego models. It has been already briefly mentioned on the 'Technic Tips' page, but it deserves some additional attention as it is often overlooked by the newcomers. So, the principle says you should have at least a general idea about how large will the model be, even before you have assembled the first two parts. Ideally, this idea will be clear and accurate enough to guide you in choosing the correct part sizes already from the beginning of construction, but often it is not necessary to go that far, as the precise scales will crystallize on-the-fly.
However, determining the scale of a desired model is not easy ― it depends on many factors. Generally, as a rule of thumb, the model should be the smallest possible as long as it allows all technical, decorative and other elements you want to implement.
In certain themes and models, the scale will be predetermined according to some "fixed" factors: e.g. a Lego City garage will have its scale preset by the figures and standard-size cars. However, in many other situations you will have a choice. This choice can be quite wide and vary according to the complexity ― great examples are the original Lego Technic cars that range from small functional go-karts to XXL supercars packed with features. However, you will notice that all these cars (as well as most other models) are tightly packed with features, and usually have no unused space at all. That is the guideline worth sticking to.
Of course, for any desired design and a list of desired features, there is a range of possible sizes that will work nicely; for example, a Technic car with a 2-cylinder engine, steering and trailing suspension could be super-densely packed into cca 24x12 studs chassis, but it may look fine in 32x16 format as well, especially if there will be some decorations. However, going beyond these extents will usually lead to dead-ends sooner or later, and get the builder back to the drawing board. Here are some headaches that might arise:
If the model is too small
▪ difficult to build sturdily
▪ likely collisions and limitations between moving parts
▪ inelegant mechanics
▪ strong limitations to the spectrum of usable parts
▪ complicated decorations, if at all possible
▪ difficult to modify
If the model is too large (much more common!)
▪ excessive weight needs extra reinforcements, in turn again increasing weight
▪ final model might look "poor" with features
▪ increased forces put parts under unnecessary strain, often risking breaks
▪ number of required parts increases dramatically, long construction and deconstruction
▪ often boring to build (many routine components)
Learning to judge the optimal size is a complex and subtle process that takes some experience, but paying particular attention to that parameter on some great models will certainly help just as much.
That brings us to the second point ― why are some systems just impossible to build without specialized parts (whether are they available or not). Namely, there is a certain minimum size that a mechanical system with a specific purpose can have. For example, it is perfectly possible to build a double-wishbone car suspension with steering and axle using just regular parts, but even in the smallest possible design, it could not occupy less than, let's say, 8x10x10 studs hypothetical block. With such dimensions, a whole car would be as large as a typical cocktail table. The forces in such a huge construction would be very difficult, if not impossible to handle by our standard beams.
Realizing the problem, Lego has arrived with a solution: dedicated parts that provide the suspension functionality in a smaller package, allowing the whole car to be within the dimensional constraints acceptable for plastic beams, axles, etc. The same can be said for specialized cylinders and pistons, differentials, gearboxes, and similar parts: they all could be made with the usual bricks, but just too massive and impractical to fit into reasonably (or bearably) large vehicles. Admittedly, some clever Legoists have built rather small and advanced components such as automatic gearboxes, but many remain out of reach, except as isolated showroom models.
But on the other hand, the available constraints are still quite generous, so don't despair. After all, sets 8880, 10188 and 8421 are out there alive and well, so if you can cram your mobile idea in some three or four thousand parts, you'll be just fine. And of course, if it's static, the only limit is the... ceiling.