On a PC, I think you have to click on OPTIONS to change the label size.
Word and WordPerfect both assume that everybody buys Avery brand labels. If you have Avery labels, the label model number is right on the sheet of labels. If you know the Avery model number, it's really easy. You don't select a label size - you select an Avery model number.
If you don't have Avery labels, it's probably more difficult.