Best gear for gravel? Tire Size? Rim Diameter?

I'm in the process of trying to get a large portion of my commute
home
off of the roads. So far I've found quite a bit of abandoned RR
tracks and such that go pretty much my way. However, I find that in
some areas with very loose gravel or sand I get slowed to a near
walking pace. I'm thinking lower tire pressure and maybe wider tires
would help me float over this stuff. Am I on the right track?

Clearly there would be a penalty on the road portion of my ride, but
if it's the difference between walking speeds and the middle ring for
the slop and gravel portion of my ride it should be a worthwhile
trade.

Since the last large leg of my ride will be on the road, I'm
considering trying very low tire pressure next time, with a couple
extra CO2 cartridges to bring me back up to pressure once I get stuck
back on the pavement.

Lastly, those that love the 29ers sing praise of them "rolling over
anything". In the past I have not put much stock in this since I'm
more the type to come out of the saddle and lift the bike over
obstacles than I am the type to try to roll over them. I'm wondering
though, would the larger diameter tire allow more contact area, and
therefore make it easier to ride through the gravel? If so, how much
easier? I may have to take one of these for a demo @ the next big
NEMBA gathering.

Thanks,

Dan