I suggest it should be mandatory to indicate the parametrics of uploaded things. It's hard to tell the dimensions from an image and whether it proportionally suits
There is no guarantee that any model downloaded from this site (or any site) is "correct". Members of this site have different skill levels. And the reason(s) for creating each model vary.
I'm always happy to find a model I can use. I download it, check some rough dimensions based on my needs, and get back to work.
How would someone "certify" their Iron Man Helmet has the correct parametrics?
A site like McMaster.com is usually pretty good about having accurately sized models.
3dcontentcentral is pretty lame, but they do/did have Vendor Certified models for some product lines.
Still, you can send your suggestion to Stratasys as a "site improvement",
Renderers can probably do more to indicate it's proportionality.
Include a reference model in the renders, like a US dime or quarter, so it's apparent that the model is dime or quarter sized. Many companies use "augmented reality" to visualize proportionality.
With photography, replace a 1:1 printout arranged with other size-indicating objects with a render using image post-processing software.
Render with scale-indicating background imagery.
Seems like a non-issue with regards to cad users, as anything downloaded that is not close enough to the correct scale can be re-scaled in CAD to the appropriate size.