I think standardized scales is a great idea, but remarkably difficult to implement. Because of the HUGE variation in size of sci-fi subjects it would be necessary to have several standard scales. To illustrate my point, let's start with some of the largest subjects.
Pretend that a major manufacturer is producing kits of some large scale topics; Super Star Destroyer, Babylon 5 Space Station, and B5 Vorlon Planet Killer. I chose these because they are big but not too big. Assume that the size of the largest model, Vorlon Planet Killer, will be 1 meter long when completed. That would be a BIG kit... The scale would be 1:45,000. Here we already have a problem. At that scale the next largest kit, Super Star Destroyer, would only be 28 cm long (about 11 inches). Everything else becomes miniscule after that.
If we completely discount the Vorlon ship and use the SSD as our starting point and make *it* 1 meter long, then its scale figures out to approx. 1:13,000. We're still okay here because the next largest model, Babylon 5 Station, would come in at around 65 cm (25 inches). We begin to run into problems when we switch to Star Trek ships.
At the same scale of 1:13,000, which worked for the B5 station and the SSD, the Next Generation Enterprise-D would be only 4 cm long, less than two inches. If we use 1:1000 instead for Star Trek then the Ent.-D goes back up to a respectable 64 cm.
I like 1:1000 for Star Trek capital ships, 1:10,000 (approx) for much larger vessels, and 1:72 for anything the size of an AT-AT or smaller. Not that much is available this way, but I'd like to see more...