I like this!
Personally, I'd argue your ships should be a lot bigger - about twice the size. First, because you're intentionally going with sometimes-terrible designs against the aliens, so you need a little more tonnage to compensate. Second, and much more important, these are WH40K ships.
According to
this chart a
Galaxy-class starship from Star Trek is about 6500 tonnes. (I'm going with low estimates here not to blow everything up.) Further, let's assume it's roughly rectangular, in the same ratios as an Imperial WH40K ship. This is wildly untrue, but it drops the tonnage estimates of the Imperium considerably, so it's a very favourable assumption.
Finally, a
Galaxy-class is 642 meters. Let's bump up to 650 for ease of math; that means that for every meter of starship you add to the beam, keeping the other dimensions constant, you add 10 tonnes. If we assume a 1:1:6 ratio of width:height:length for these starships (a figure I arrive at by holding my fingers up against a picture of a
Dauntless), and we assume constant density (and, of course, that the Federation and the Imperium follow identical materials engineering practices), then we can get comparative tonnages based purely on the beam length, which is our available information.
The
Dauntless is 4500 meters, which weighs in at a little higher than 950,000 tonnes.
Clearly that would be absurd to try to replicate in Aurora, and I don't mean I think you should give yourself that much more tonnage. But I also think it would be really cool to see how really big ships (100kt and higher) act in C# what with the changes to shock damage and so forth.