Those are "back in the day" numbers. There aren't any realms currently that break 40 people. I would put the limit at 25. The seems to be the average that the "larger" realms have currently.
Also, what if all the realms are either above 25 and below 10 nobles? New players can't join? I think only one limit can be applied. Let new nobles join small realms. Maybe they get bored by not much going on, or maybe they breath some life into the realm. Who knows.
Definitely should keep the size limit on the merges. Huge behemoths, as stated earlier, do nothing for the game. Mergers aren't even supposed to be allowed, they were introduced as a way to amicably increase the size of two small realms. I don't think we should go throwing more freedoms at it.