How do I go about setting up a Pro Tools (or any DAW) session for a live recording, when there is not a fixed tempo, time signature, etc.?
Since you probably won't be needing/using a time grid for anything during mixing, you shouldn't even need to worry about those settings. Just record and let it fly.
If I record at 44.1, how much hard drive space (approx) will I need for a 2 hour show?
The general rule of thumb for 44.1k WAV is approximately 10MB per minute per track. Meaning that if you recorded 4 tracks that would be 40MB/min. Multiply that by 120 minutes, and you have 4800MB. Just to have a safety factor built in, round that up to 5GB. Of course if there's a set break or two in there, that'll shorten the time (see below.)
Should I pause the recording between songs and/or during a break?
Let it roll between songs. You can always edit out extra stuff between songs in post if you want.
Set breaks are a different story; no need to record those. Just make sure you don't stop recording until the artist actually puts his instrument down. And either start recording early, like when he first picks the instrument up. Or, arrange a signal between you two to make sure you guys are both ready for each other.
How much or little compression should I use on the vocal?
That totally depends on how the vocal sounds and how that differs from how you want it to sound. I'd recommend waiting until mixing to decide on that,and use compression or limiting only if the singer has lousy microphone technique and lousy voice control and you need to limit the signal to keep the signal level from going too high to fit without unpredictably clipping at uncontrolled points.
Should I use a DI as well as mic the amp, affording myself more options later during mixing?
As long as you have the gear and the extra recording channel to fit, there's not many good reasons not to give yourself that option. Go ahead.
Where should a room (ambient) mic be placed - on stage, in front of stage, back of venue?
You'll get enough room reflection in the stage mics to make it sound live enough. I'd skip the ambiance for now and concentrate on the audience applause. I'd stick a cardioid on stage side pointing back towards the audience. Then in post, I'd just bring it up during the applause and back down to the background or less during the songs. YMMV.
G.
