any limits on mechanical seals?

Suppose you have a large space station (OK, let's say a colony) with a
rotating portion and a stationary portion, both pressurized, and with
constant traffic back and forth between them. Obviously you need a
large mechanical seal between them, and I have some questions about that
which I hope someone can answer:

- In general how likely is this to be a problem? Suppose we want a
corridor 20 m wide between the two portions -- how much bigger is that
than any seal we've built before?

- How can I estimate the leak rate through the seal?

- Can I characterize the problem only by the pressure difference between
the inside and outside? In other words, is a seal that holds 1 ATM
against a vacuum the same as one that holds 2 ATM against 1 ATM?

- What type of seal would you expect this to be? Radial shaft?
Labyrinth? Rotating face? Something else? (I only vaguely understand
these different types of seals, so even small insights will be
appreciated.)

Many thanks,
- Joe