There is not a specific best size to Flyers and Leaflets, it depends on their purpose:
105 x 148mm (A6): One of the most popular sizes of flyers is A6, is the smallest size great to short and direct campaigns.
99 x 210 mm (DL): Fit perfectly into DL envelopes, making them a great choice for a direct mail campaign.
148 x 210mm (A5): With intermediate size, half the size of the A4 and double of the A6, is one of the most frequently used dimensions.
210 x 297mm (A4): Larger size, suitable promotions containing images and illustrations of products and services.