Go to Solution.
You can not do that in layout designer, but you can do that in customize data layout
on the print tab, check the bullet for Use specified printer and settings ... and change to landscape
then click the layout designer button
View solution in original post
Go to Lists menu > Templates.
Edit this template.
Go to Additional Customization, the Print Tab.
Change the setting here, to "Use Specified Printer Settings below for this Invoice."
Now, change the layout here, using the Radio button for Landscape orientation. You can even custom size this on half-pages.