Did you try this, yet?
In Pages (page layout mode) -
Make a text box 5cm x 5cm
The Metrics tab in the Inspector window provides you with a means to create that box at exactly 5 cm x 5 cm.
Click on the graphic tab in the inspector, where you can add a box around that text area, using Stroke. You can make it a simple line if you want, or a happy-looking frame of some kind....
Fill it with text, or graphic
Print it out.
You could put about 20 boxes on one standard page, I suppose.
Cut to the size, if needed.