Rachel in Lacerte Support found it for me.  Thought I would post it in case anyone else finds my question when searching for this answer...

It's under Settings/Options/Tax Return.  In the top section of that screen, there's a dropdown at "Schedule K-1 Mailing Slip Sheet" where you choose Portrait, Landscape or Large window.  (Same choices for Tax Return Folder and Return Mailing Envelope slip sheets are at that screen, too)

Would be great if that answer could be found easily with a search of the knowledge base online or in the help function.