Wookiepedia and other sites all say ICS is the source, i have not seen this figure stated in ICS? Where has this figure come from or how was it extrapolated? Feedback appreciated! (:

Also any information or calculations on ship or reactor volumes would be appreciated, as would be for fuel tanks.

ICS says that the Venator's "main reactor annihilates up to 40,000 tons of fuel per second at maximum power," which is equivalent to 3.6e24 watts. On the Separatist Ships page it says that 4-6 Recusants can outgun a Victory or Venator, which means they must be similar in reactor output.

As for reactor volumes, ISD's reactor is ~140 meters in diameter, with multiple subsidiary reactors, Acclamator's is ~72 meters, Venator's is 80+ meters with a sizable bulge, DSI's is 28.8 km, and DSII's is 58.5 km.

There are some ship volumes in this post: viewtopic.php?p=2582176#p2582176
I can't vouch for the accuracy of some of those figures though.

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Have a nice day.

Some of those numbers were provided as massing models, so they are representing the upper bound of the volume that can be encompassed by a sphere or a cylinder in the hull. There is obviously armor and other systems that would reduce these numbers, or in the case of larger vessels of unknown arrangement many subsidiary reactors that could increase it. That series of posts was an attempt to scale power output to reactor volume assuming a fixed efficiency, equivalent operation and fuel density. I'd call those numbers rough in many cases since we only have a few cut away or solid numbers to reference to.