Regarding size ranges- one thing that occurs to me is that some cruisers may be much boxier, with a higher width-to-length ratio than others. For example, some of the big 'cruisers' have those big nova cannon: those sound like long-barreled axial weapons, and you may need a long, spindly hull to build the nova gun into the keel of the ship. The long narrow hull also favors bow-on engagements, because it means you present a much smaller target to the enemy. And you can save mass on your main armor belt by putting it perpendicular to the axis of the ship, like the gun shield on an artillery piece.
(It doesn't have to stick out like a gun shield, it can just be a solid "pie plate" of armor that separates the expendable stuff near the front end of the ship from the important stuff near the back, built into the hull where you can't easily see it).
Whereas something designed to launch broadside barrages of torpedo and what Battlefleet: Gothic models as "weapons" or "lance" fire might well be built shorter but with a fatter hull, so that it can wrap more defensive depth of armor and shielding and expendable spaces (like crew quarters and fuel bunkers) around the core hull.
Possibly. We know that the Essene from Eisenhorn was 3 km long and 700 m tall (which was basically as large as older cruisers) whilst the FFG cruisers were 5 km long and .8 km at the fins, so there is reason to believe that 'length to width/height' ratios vary. Given that engines and prows (or lack of prows) has been a significant contriubtor to lenght (engines make up something like 1/3 to 1/2 the length/mass of a ship, depending on source, so putting in a different engine system would adjust the dimensions of the ship.)
Fudge it a bit further by differences in technology, consturction, etc. from sector to sector/planet to planet to permit individual differences in design and that might account for most differences.
This actually makes a lot of sense if life support and routine sensor watch use less energy than firing up the main engine. We normally think of high-power SF drives as being continuous boost, but there's no logical reason to do that if it doesn't pay.
For a continuous-boost spaceship with high accelerations, adding an extra hour of engine burn time gives you diminishing returns in saved travel time. The first hour gives you a fixed travel time, say 60 days. The second hour gives you a travel time of 30 days (one half that). The third gives you 20 days (one third), and so on.
So does it make sense to burn the engines for another hour, taking off component lifetime and "expending a prodigious amount of the Emperor's fuel
," just to save five days of trip time?
It might not, under a variety of conditions. Say, if you aren't confident of being able to refuel and your tanks are low. Or if you might have to fight a very energetic and dangerous battle at the end of your mission. Or if this is a routine redeployment of troops or ships and you aren't in any real hurry to get things done. Or you aren't totally sure of how long your century-old most Omnissiah-blessed rocket engines will hold up under a continuous 50g burn and don't want to risk blowing a gasket before your next scheduled maintenance stop.
So maybe 120 hours in transit is a smarter option than spending 1 hour burning the engines at top acceleration. Unless you're in a hurry.
Could be. again I probably wouldn't generalize as I'm sure there are some contradictory examples. Given the variation of technology (STC standardization only goes so far) we could envision a variety of engines with different performance criteria for givne hulls. Rogue Trader (The RPG) tends to go this route with their ship design and constriction, and we also know of Battlecruisers in BFG (which were cruiser hulls with IIRC battleship engines strapped on) an the Voss-pattern vessels (which were light cruisers that had shitty engine performance, but could compensate with firepower and I think durability.)
One of the things tht has sort of been glossed over in more recent editions is whether or not 40K ships carry onboard workshop/fabrication facilities. In Space Fleet, all ships had them to build the stuff they needed to repair and maintain it in the field, but as warp speeds have gone up (since BFG era) the transit times and times 'away from base' seem to have gone down (months instead of decades.) the BFG rules mention that there are workshops onboard for building/reparing fighters but that's it. and FFG's Rogue trader stuff implies that more ships (at least in Calixis) get my on stores.
In all probability its handled on a sector by sector basis, with battlefleets customized to the roles and enviroment they operate in. That means some fleets might be designed for long-range, independent operation (which can have implications for their invidiual capabilities including engine performance) whilst others might be designed to return to base more often (weeks or months away from base.)
Crew complements are another area where 'variation' exists.. cruisers and battleships carried thousands or tens of thousands per ship (and escorts I think hundreds) earlier on, but thats gone to tens or hundreds of thousands average with FFG material.
Of course, there's a great difference between sustained barrage and individual weapon capability.
At the Somme, the British fired off roughly one and three quarter million artillery shells, easily translating to a few thousand tons of explosives. This does not mean they managed to do as much damage to the German lines as a couple of low-yield tactical nuclear warheads would have done.
The same comparison might lend itself to a seven-day bombardment from shipboard energy weapons, versus a few shots from something like a nova cannon.
That's one inteprretation, although we're probably talking minutes worth of bombardment (less than an hour) rather than days. On the other hand some sources have indicated that ship to ship encounters are mostly manuvering for position and to make single, brief broadside passes. In that context a turn could be mostly moving about for advantage, followed by very brief barrages being fired. Or maybe multiple barrages.
Some people I have known believe that macro cannon shells take upwards of half an hour to hit the target across tens or hundreds of thousands of km. I haven't figured out how that's supposed to be (with unguided shells.) either.