Great info from everyone so far! I use Beersmith now, but I did use Biabaccus before that.
To Feldman's point- have a plan for draining the grain. I use a ratchet hoist for 10gal batches, but I always was able to hold 5 gallons. It can get a bit messy/heavy, but you'll be fine!
If you're milling your own grain, then I'd recommend that finer is better.
I always try to hold back a gallon or two from the strike water, and I'll pour that over the spent grains. This helps improve efficiency and it gives you more room in the pot. If my calculator said to mash with 8 gallons for a 5.5gal yield, then I would actually only mash with 6 gallons. Then when I remove the bag (hoist above pot), I pour the remaining two gallons over the bag and it drains into the pot. This is a bit harder to do without a hoist. Many people will "dunk sparge" by dipping the bag into a pail of water, then adding that water to the boil.