For BIAB I would suggest getting the biggest vessel possible so you will be able to do a full volume mash. I have a 49L stock pot and find I have 7.5cm of head room with a low gravity beer (1.038) for 23L into fermenter. Holding back water is fine but just a bit of a pain as you will need a second vessel.
BIABrewer.info is by far the best place to read about the BIAB technique and equipment go check it out!!! also the BIABacus spreadsheet you can download for free is probably the best tool around for beer recipes, it can calculate and estimate all the volume for you.