How easy or hard your task will be depends on the paper stock you are using. If you are purchasing card stock that is smaller than normal paper, that's pretty easy: measure the cards and then open Pages and select a blank page layout template, select the Inspector, click on the first tab, and then click on the Page Setup button. In the Page Setup dialog click on the Paper Size popup menu and scroll down to custom. Create a new paper size by clicking on the plus button - name it Invites or something like that. Set the paper size and leave margins as they are. Back in the Inspector, set your document margins. The Pages document will magically change to the size you set. Create your invitation and print.
If, on the other hand, you have bought Avery card stock, you are going to create your own Pages template. You'll need to do careful measurements again and open a blank page layout template. Make sure Pages is showing the rulers. You'll need to drag out guides to define the actual page. In other words, let's say the card starts 1 inch from the top of the card stock. Put your cursor up in the top ruler, click and drag down an inch. This will drag out a blue guide which you'll use when you create your text box. Drag out the rest of your guides. Then insert a text box and drag it out to fill the blue box you created with the guides. Now add text, graphics, and print. Oh, print first on a normal sheet of paper and then hold it against a piece of card stock to see how it will fit on the real thing. Adjust as needed.
