This article applies to Labeler version 6.

After opening the .LAB file of choice, use File > Page Setup's "New" button to create a new layout. Click on New and name the layout something descriptive, then you can define how many labels Across, how many Down, label size, page margins, and space between labels.

Then go to the Printer Setup Tab at the top and where it asks for the paper size, change it to Universal, then specify the Width and Height of the paper itself.