The best postcard size for direct mail depends on your specific marketing goals, the content you want to convey, and your target audience. Different sizes can have varying impacts on recipients, so consider the following factors when choosing the best postcard size for your direct mail campaign:

Content and Message: The size of your postcard should accommodate the content you want to include. If you have a lot of information, images, or graphics to share, a larger postcard might be more suitable. Conversely, if you're focusing on a simple and concise message, a smaller postcard could work well.
Visual Impact: Larger postcards tend to grab attention more effectively due to their size and visibility. They offer more space for captivating images and bold designs, making them a good choice if visual impact is crucial for your campaign.
Readability: The size of your postcard should allow for easy reading of your message and contact information. Avoid choosing a size that's too small, as it might result in text being difficult to read, especially for recipients with visual impairments.
Target Audience: Consider your target audience's preferences and habits. If you're targeting a tech-savvy audience, they might appreciate QR codes or interactive elements, which could influence the postcard size.
Budget: Larger postcards may incur higher printing and postage costs due to size. Be sure to factor in your budget when choosing a size.
Design and Branding: The chosen postcard size should align with your brand's aesthetics and design. Different sizes can convey different feelings and styles.

Common postcard sizes for direct mail campaigns include:

4" x 6" Postcards: This is a standard size that's cost-effective for printing and postage. It offers enough space for a concise message, an image, and contact information.
5.5" x 8.5" Postcards: Slightly larger than the standard size, these postcards provide more space for graphics and information. They can be more attention-grabbing and allow for a more detailed message.
6" x 9" Postcards: This size offers even more space and impact. It's suitable for campaigns with more information, images, or special offers.
6” x 11” Postcards: These stick out in the mail more and provide more room for content. If sent first class the postage rate will be higher than smaller postcards
8.5 x 11” Postcards: These postcards are a full sheet of paper. The postage on this size is significantly more expensive unless you are completely saturating an area. The printing is also more expensive but there is a lot more room for content and this will be one of the larger pieces in the mailbox.

Ultimately, the best postcard size for your direct mail campaign depends on your specific objectives and the impression you want to make. Consider working with a professional designer or marketing expert to help you choose the size that aligns best with your goals and resonates with your target audience.