I do 5 gallon biab batches in a 10 gallon pot. Sometimes the yield is closer to 6-6.5 gallons and that's ok, but at the very threshold of my bucket fermentors. That's a pretty full pot with grain included. If I'm careful at the start of the boil I could probably get 9 gallons of wort rolling for a final batch of 7-8 gallons.
Anything smaller and I just use a 4 gallon, beat-up aluminum pot we've had for 20 years. I heat it on the electric stove, mash in directly and then strain into a bottling bucket using one of my biab bags. I recycle the wort till it's clear, add hot tap water to rinse the grains a bit and to up my volume if needed, then back into the same pot. I usually boil outside on my propane burner.