I like 1 gallon batches because it lets me make a variety of beers recipes to try. It is pretty fast, both in process and clean up. I've got a few recipes now that I'd like to step up to 5 gallons, I just don't have the equipment to do a larger batch. My larger batches usually involve extract at this point.
Regarding grain bags, if the grains are particularly sticky and won't get off the bag, I pitch the bag. I get the cheap muslin kind. Not worth the hassle to me to get it super clean. If i have a few bits and pieces left, no biggie, it'll all get boiled next time so nothing can really harm anything.
Primary: JAOM, GW Small Beer v3
Bottled: 15 Minute Cascade Pale Ale, Cream of 3 Crops, George Washington's Small Beer, Sam Adams Holiday Porter Clone
TO DO/BREW: Spruce Beer