The smallest booklet commercially available, a stapled booklet the size of a business card, is a really novel print idea, to compliment your tiny product or service. Lets call it a mini booklet.
These booklets measure the same size as a business card, thats 55mm wide by 85mm high. The perfect size to fit in a wallet or purse. Just one staple on the long fold edge and coming in page counts of 8, 12, 16, 20 and 24 pages - more pages upon request.
You might also want to consider a tiny cross folded leaflet, which can fold down to the same size from one printed sheet.