Not to wake an old thread, but I just completed my third BIAB, all between 2-3 gallons. I just wanted to add my experiences the last couple of weeks in case anyone else comes across this via Google.
I've been learning a lot of water lessons these weeks. My first two batches were a lot smaller than I thought they would be after trub, and boil off has been lower than I expected. Constantly keeping track of numbers.
I made a Tripel yesterday - 1.075 OG, 7.5 lb grain bill in my little 5 gallon brew pot. I was able to get 3.9 gallons pre-boil (dunk sparged with .75 gallon, so not a "true" BIAB I suppose), and boiled down to just over 3. I lost half a gallon to kettle trub, and I can already see I'm going to lose another half G or so to trub in the primary. Next time I'll put the whole contents in the primary and hope it all settles out.
I'm disappointed to lose so much to trub, but I'm enjoying BIAB and it suits my purposes perfectly. I plan now to lose at least .5 a gallon to trub per 3 gallon batch for smaller brews, and even more for big on beers.
Happy to say, the yeast are going crazy. Biggest beer I've ever brewed, and the most wonderful looking ferm I've ever seen. Woohoo!