Don't quote me on this, but from what I've heard:
1. 160GB
2. ~100 hours
3. ~20 hours
4a. No
4b. Rumor had it some receivers had a external SATA port, but it wasn't activated. I don't know the current state of affairs.
5. No, but the ability to stream video from one DVR to another receiver (DVR or non-DVR) is suppose to be coming if it hasn't been released yet.
6. Depends on the channel. It appears some channels (nationals in particular) are encrypted and won't transfer via firewire. Other channels, such as locals, may not be.
All the above is from reports here or at DSLReports in the FiOS forum.