I mash 3-gallon BIAB batches in a 5-gallon beverage cooler, and boil in my 5-gallon kettle. Fermcap is required to prevent boilovers, but otherwise it is quite manageable.
Grain absorption is much less than traditional all-grain, especially if you squeeze the bag. I'd start with 0.125 gallon/lb, but (depending on how much/hard you squeeze) it may be significantly lower than that. Absorption is so low, that I don't waste my time calculating out a specific volume for my mash. Almost every batch uses 4 gallons exactly, regardless of the grain bill. I use an extra quart for a 90-minute boil, and 2 extra quarts for a ridiculously hoppy IPA. This is close enough to get me within a couple of points of what I'm shooting for, and I'll generally just tweak the grain bill the next time I brew if I need to.
As far as weight goes, the most I've done is 8lb of grain at a time. This is fairly heavy to lift out of my cooler at first before it is fully drained, but it is manageable enough for my out-of-shape self. I wouldn't want to do a grain bill of more than 10 pounds or so without some kind of pulley system.