Here are my figures:
B'rel: 109 m
D'deridex (Romulan Warbird): 1041.65
The figure for the Ambassador is from Rick Sternbach. He later said Mike Okuda wanted the figure of 526 to stand, because he had published that. Rick said the drawing somehow got stretched out when copied, increasing his 478.5 meters to 526. Finally, about the length, Rick just said it didn't really matter but he could live with 478.5. After changing it back and forth a couple of times, I kept it according to the original drawing, at 478.5.
I was able to get Bernd of Ex Astris Scientia to change his figure for the Wells class by showing him a scan of Rick's drawing posted on drexfiles with the size written on it, but he wouldn't change his listing for the Type 10 even after I got the intended size from Doug Drexler, the designer. who argues that it does fit into the most accurate version of the Defiant CGI. Doug told me directly it should be 8 meters, and Bernd still insists on 6.5.
But even for SpaceShipOne, a real craft now in the Smithsonian, when i looked for dimensions, I found that the Smithsonian, the manufacturer, and Wikipedia each had figures that disagreed with the other two. For the Nebula class I was lucky, because the saucer width is the same as for the Galaxy class, about which there is no argument, allowing me to ignore the conflicting figures on the Web and calculate based on measurements. The same goes for the Argo Baja buggy. I used the known wheelbase of the Toyota truck it was built from. For the inspection pod of the NX-01 and a few other things, I got dimensions directly from the Christie's auction catalog.
For the Romulan Warbird, I was able to get some help from Andy Probert. He said they grabbed the drawing before he had a chance to put in the impulse plasma exhausts and that the model builder didn't notice the omission. So even though they're missing onscreen, I added them to the MSD according to his instructions. He also said that they were wrong to show a disruptor blast coming from the main deflector. He also said that the row of bumps on top were escape pods and that Doug Drexler didn't know that when he added a small version of them when designing Romulan Sentator Vreenak's shuttle.
My wife gave up on Trek when she saw the Ferengi.