What size pot/pan should I use to make a smaller amount?

I think that depends on what you mean by smaller amount … are you cutting it in half? Then I’d say the pan can be half the size as well … 3 quart. Because you flip the timballo you won’t want it to drop out of the pan onto the serving dish so I’d say err on the side of too much filling for a smaller pan the ensure it goes all the way to the top.
As a side note, I did a bunch of research a while back, people cook timballo in all sorts of vessels including clay flower pots lined with foil!