You can see that the model on the left is "finer" (more polygons) than the one on the right. But, consider the width of the facet that I've highlighted in pink: it is four times as wide as the facet in green. When designing for a RP printer, you have to keep in mind the wall thickness limit, but you also need to consider the detail thickness limit.
When designing a model, you must keep the whole object above a certain thickness, but, you should also keep the facet size below the "detail limit." Let's say that the printer can print a detail at 0.2mm: if the large, pink facet is 0.4mm and the finer green one is 0.1mm, then the pink one is going to show up as a flat spot on the model, and won't look curved. The green one will look just fine.
Why does this matter when enlarging? Because when you take the model on the right with less polys and blow it up by four times larger, you'll end up exaggerating how blocky it looks. I personally try to model at somewhere around 1/2 of the detail limit size, but that means my models won't necessarily still look good when enlarged. The only way you can overcome this is to split every polygon into smaller pieces, which sometimes is as complicated as drawing a whole new model.
What about scaling down? Shrinking the size presents a different problem. Again, wall thickness limit comes into play. If a segment of the model is 1mm thick, and I reduce the overall model by a factor of 4, that segment is now 0.25mm thick, and it is too thick to be printed on the printer. Here, again, you'd have to re-design the model to account for the different thickness limits.
Scaling can be done quickly, but remember the impact of polygons and price. For miniatures, detail is crucial, so while the whole object does have to be above the wall thickness minimum, keep the facet size should be below the "detail limit." Similary, when enlarging a model, blowing it up will exaggerate a 3D model's blockiness.We hope that over time, more and more materials with cheaper prices will become available. In the meantime though, it's helpful to keep pricing in mind when scaling your models up. You can save on cost when enlarging your 3D models by maintaining the wall thickness limit, instead of exceeding it by too much as you scale.
Visit Shapeways material guidelines to check out the wall thickness and detail limits for yourself, or if you're ready, you can upload your model to check out the sizes and printability in all materials immediately.