1) Yes you have to tie it in to both manifolds.
2) Size depends on where you want the power in the RPM band. Smaller = lower end power, and larger size means little or no extra power in the low end and more in the midrange to top end. Turbo's have to be sized very carefully based on engine displacement vs. powerband.
Best to do some serious research. Just "having an idea on what all i need" isn't enough. This isn't an insult but hard truth: If you are not sure what size turbo is a good size, you don't really "have an idea of what all you need". Turbocharging isn't a "rough idea" or "good guess", it's an exact science.
Many people use a T3/T4 hybrid on a 3400 for streetable power. Some of us are running T70 for less streetable and more race power.