Does the ISD have to be that massive? With the Vic already being approximately 5in, an approximated ISD at 9in would be proportionately accurate but also somewhat unwieldy on the table. Remember the base, with shield dials and arcs of fire, would have the footprint the size of a large hardback book. That much base and such a large model would, even if accurate, spoil some of the immersion to me. I find myself thinking that the ISD just needs to be easily recognizable as bigger than the Vic. Similar to the way the Vic is bigger than the Gladiator. Once it passes the casual eye test, that's enough. Because as goes the ISD, so goes eventual ships of comparable size like Rebel Mon Cal ships. We obviously want ISDs and Mon Cals duking it out. And they need to command the field as the largest ships or at least the typical large ships. But they also need to be manageable. "Excuse me a moment while I bump Home One trying to move a squad of TIEs on the other side of the table." Likewise those mini rectangular bases on the current ships are both functional and innocuous. Imagine multiple bases the size of hardback books on the table ruining the effect, like being able to see strings holding the Death Star up in the movies. I know many people want everything to scale. That's my preference too. But, given how much larger it is, price costs, the need for a table that can be managed easily without fear of knocking over multiple huge models with a complex movement or firing range tool, and how massive the bases would be, I'd be happy with a relative scale ship that was simply larger to the naked eye.
But really, this may have been an excuse to post more images of capital ships on a slow night...