I keep a bar of soap, wrapped in a wash cloth, inside one of those plastic mesh fruit bag thingies, with a little zip tie closure. It's in a ziplock bag in my BOB, but in my hygiene kit not the FAK. Even a small bar has always seemed to big to me for a FAK, when there are other cleaning options available.

For example, I read the other day about washing poison ivy off with rubbing alcohol rather than soap. More effective, apparently, and alcohol pads seem like they could save space in the FAK over soap and water.