How do we know Voyager is 344 meters? Was it ever stated on screen at any time? Or did Rick Sternbach tell us in a post?
Also, was there ever a size mentioned for the Prometheus-class?
For one thing, it's stated in the Voyager Technical Manual
, by Michael Okuda and Rick Sternbach, put together for production use in 1995. You can view it online here, and the length of 344 meters is given on page 11.