Hi Palle and welcome!
Batch size is defined by the program as the amount of wort expected to be transferred to the fermenter. This is post-boil and cold wort volume.
If you are using a bag and still managing your mash as a traditional mash liquor and sparge system, then you should treat it as a traditional mash liquor and sparge and use that profile. BeerSmith defines BIAB to mean a full volume mash, as this method was originally developed. If you are using a standard amount of water to sparge, like always sparging with 4 liters of water, then you can still use the BIAB mash profile and set up your equipment profile to 'top off kettle' with the 4 liters you intend to use as a sparge.
I hope this is clear, if not let me know what part is not.