The maximum size will be the maximum allowed for one side in the Stronghold battles, so initially 30. You will be able to set limits though, in case you specifically want to do Outposts (15) or Hives (5).
Note, however, that if players don’t go for skirmish matches, it means they have a much lower pool of potential opponents to fight against.
If you start a 30 strong warparty and jump in the queue, then 10 other people come along in your faction and 10 people from another faction those 20 people will be matched together. The system optimizes for getting people into a match as quickly as possible, so if a match can start, it will. The system does not optimize for getting people to have been waiting into a match before other people who haven’t been waiting.