The nobles in the chapter on Brevoy are the King, the Lords and the Lord Mayor. The kingdom rules mention Barons and Dukes.
I'm thinking that Lords are the highest landed nobles, equivalent to Dukes in other kingdoms. Below them are Earls and Barons, also landed, hereditary nobles. The Lord Mayor is a special honorary title outside this system, generally taken to be equivalent to Earl (although some Earls and Barons naturally contest this). Non-landed titles include at least Lord Knight (non-hereditary, granted for valour or military service) and Lord Steward (non-hereditary, for high ranking administrators).
I'm planning on not using the size based titles in the kingdom rules. Instead the PC's "kingdom" starts as an Independent Barony, owing featly only to the King himself. (This is the loophole. If House Rogarvia is no longer on the throne, they are no longer bound by their oaths. Usually oaths are to the Dragonscale throne.)
Many Earls and Lords oppose this viciously, House Surtova foremost. Once they annex Varnhold (another such barony), the King names the combined land an Independent Earldom, also giving them a right to grant a limited number of titles to raise vassals. (This further inflames the already deteriorating relations between north and south.)