I do 10 gallon BIAB in a 15.5 gallon Keggle (62 qt), and do my 5 gallon batches in an 11 gallon kettle (44 qt).
My starting water volume on my 10 gallon Citra APA batch last weekend was 13.6 gallons for 20 lbs of grain, and everything fit just fine with a good bit of room to spare. 19 lbs of two row and 1 lb of honey malt displaced about 1.6 gallons, so I was up around 15.2 gallons (including the displacement) of the 15.5 during the mash, then back down to around 12.5 gallons pre-boil for around 10 gallons post boil.
That's about what you need for an average gravity batch (this APA ends around 5.6% ABV).
Hope all that helps!