When i saw the VSD at GenCon, i estimated it to be approx 5"-5.5". I never got it in my hand though, so take it as an approximate.
An Imperial class would probably be about 8" long based on their sliding scale.
And we can assume like X-wing that the "small" ships will have one base, and the "large" ships (MC80s/ISDs)will have a bigger base if needed for stability/game balance if/when they are released