Logos are how customers remember your brand. Therefore, it’s important to use one that is clear and crisp in your emails.
We recommend for your logo image to have at least 72dpi. This dpi rate is different than actual pixel size. For logo pixels, we recommend around 170 x 40 pixels.
You can also use a logo larger than this size, and then use the 'image width' field to make the logo show smaller, but the actual image size is larger.
Please take a look here for more information on logos.
I can also suggest using an image block for the logo, and then a 'links only' header bar below. So, the logo image is not within the header block.
Additional Resources: