When I did V'ger I first tried canon size but QB hung and would not load.
actually, in my version of the game i scaled up V'Ger to be the Cannon size of 98km long
the game doesnt lag at all and there are no loading problems
i have also scaled up the death star to be 160km (as it should be) and there are no loading problems there either
by the way, the executor is 19km long and you could just use the KM1.0 rescale option to make one the actual size
take the one available on BC files and rescale it to be 1.8 times larger, that is the accurate size compared to all other ships ingame