If I buy it with tokens, can I play it RIGHT NOW!?
Yes.
Is there a large player base? Is it easy to find a multiplayer game?
So-so -- most people are in the SDC chat channel, not the in-game lobbey, and it can (sometimes) be hard to find a game. For example, I have a really hard time because most of our players are east coast, and I'm west.
How long do multiplayer games last and what is the typical galaxy size?
It varies,
wildly. Plan on ploppping down an hour, minimum, but they can last a
very long time past that, especially if players are equally skilled.