Yes, attachments and images in the email as well as the overall size of the email itself are subject to a limit. The following general limits apply:
Templates wider than 600px may create problems when rendering in your subscriber's email client as they might have to scroll across to view the entire template. We recommend that your template is not more than 600px wide. Height is not restricted as users prefer to scroll up and down than left and right. Keep in mind, though, that if your newsletter is too long you might lose your readers half-way through. Try to keep your content as concise as possible.
Dimensions for images should never be bigger than the template dimensions. Keep all images at the lowest resolution acceptable for the web. We recommend keeping image sizes below 25KB and the image format set to low resolution (72dp RGB) to decrease the time it takes to load images in the inbox. The higher the resolution, and the wider and longer the image, the bigger the size. Big images will probably be blocked by the receiving server, and they may take too long to download or render.
Images under 300KB are acceptable, but try not to exceed 750KB. The system allows you to upload attachments up to 3MB, but attachments this large will probably be blocked. Keep your attachments as small as possible to improve delivery.
Aim for an overall email size of 100KB. The closer your email is to this recommended size, the more likely it'll be accepted by receiving servers. Aim for a balanced image-to-text ratio, this is best practice in terms of e-marketing. Most servers can't ‘read’ images or identify what they contain, so receiving servers may interpret emails consisting only of images as spam. A balanced HTML email is more likely to be delivered.