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.