How can I quickly reduce my realm size?

Related to how to find out how big an opponent is, I just found out that I'm the biggest kid on the block: my realm is bigger than even the Holy Roman Empire, so I can't invade anybody. But my realm size is only about 20 bigger than the HRE; how can I quickly reduce my realm size so that I qualify for an invasion?

Bonus points if I can quickly "recover" any bits of the realm that I lose.

Give demense holdings to your heir, then make him independent. When your character dies, you should have it all again. Also note what svick says in the comment on the question above:

You can always grant independence to some of your vassals. And if they are part of your de jure realm, it should be relatively simple to get them back (but I haven't tried it).

You can just request them to be your vassal, or press a du jure claim on their territory after you want to grow again.