You can do 5.1 in a 700MB file, the problem there is you don't want to have the audio eat up too much of the bitrate. But of course you already know that, which is why we're having this conversation anyway.
I'd suggest doing it first as a 5.1, and if it ends up looking shite, you can always re-do it as a 2.0 (or 2.1, but at that point the bass channel would be kind of unnecessary in my opinion).