Some time ago, I actually put some thought into an "evacuate Earth in 6 months" scenario, and I came up with numbers that were staggering. Considering a population of 6 billion (6e9) people to be evacuated in 180 days, you'd have to put close to 1.4 MILLION poeple into ships PER HOUR. Nonstop, 24/7. Putting my record planet population in GalCiv (21bn) into a colony ship in 1 month would push that number to a little over 29 million/hour. So even if you could build a ship large enough, you'd have to build it all doors. Perhaps it'd be easier to just stick a thruster to the planet?

Space sharks are easy in comparison.
Nowlet's consider just for fun that each person would need 2 cubic meters including all life support on that ship (we'll ignore stuff like generators, engines or storage for now). You're now thinking of a volume of 42bn cubic meters. Now let's think Borg and make a cube of it. That cube would be about 3.5km long in every dimension, but much less comfortable than a Borg cube since you'd have no space to move and no food. Consumables are another book, so let's just freeze the inhabitants.
Some things are perhaps best left unpondered... thinks like a 3.5km cube full of frozen humans. Unless you're a Drengin, then you might be VERY grateful for that mega-snack.
~SDC~