There's no canonically listed size for the Centurion; when I did the craft's stat block for the RPG, I had to estimate it based on its length in pixels versus the length of the Broadsword (a craft in the same game for which there is an canonically stated length) in pixels.
You shouldn't need an "official" length though. You can get a program like GIMP or Photoshop and measure the pixel length from each of the three perspectives, which I think is in the Privateer Reference Card? Will definitely be available from the extras at GOG and I'm pretty sure they're also available via WCPedia. You can measure length versus draught and length versus wingspan seperately, and then get make whatever adjustments are needed if you get a different length measurement between the two. You just won't know what the actual scale ratio of your model will be.
For the record, when I did my calculations I came up with a final gross volume (length x width x draught, not a particularly accurate measurement of the actual volume) of 17,998.75 m^3.
EDIT: Actually found my notes on this - the Centurion comes out to 462 pixels long, 436 pixels wide, and 137 pixels in draught, based on the (probably correct) assumption that it's to scale with itself.