Critical Systems:

wikipedia does a better job than I on the definition of a critical
system although I'm not suggesting that everything in this article
refers to a potential software system but it's a good illustration.

http://en.wikipedia.org/wiki/Life-critical_system

Real time systems:

An example of a real time system in this context is
an odds arbitrage back end publishing to the WWW via a
collection of web services, of course this is just one possible
interface, it could publish to another computer system or another HCI
This requirement of course implies a complete decoupling of view and
implementation. For more on odds arbitrage

http://en.wikipedia.org/wiki/Arbitrage_betting

It's 'real time' because publishing incorrect odds could cost someone a
great deal of money, the odds need to be available for manipulation by
the arbitrage engine as soon as they appear on the relevant bookies
website (in fact I've had them appearing _before_ this as I managed to
process them before the bookies web site did :-).

http://www.python.org/about/success/#real-time has a few examples
but I'd be interested to hear 'from the horses mouth'

Again, wikipedia is your friend
http://en.wikipedia.org/wiki/Real-time_computing

What is the largest Python project you have experience of, you can use
any metric you want, a simple KLOC, function point or cost analysis will
be fine.

This is a genuine enquiry and not designed to 'diss' Python in any way.

Many thanks