Hello sbz2,
Thanks for writing in.
Actually, this is something MailPoet 3 already does. When choosing images via the WordPress Media Manager, MailPoet will pick an image size out of available ones, that is closest to 1320px in width – which is twice the size of the column width of a newsletter (660px).
And then the 1320px image will be constrained to be displayed in at most 660px width.
However, in order for this to work as expected there are some prerequisites.
1) Your theme must not define a $content_width
global variable, or it must be sufficiently large enough, as any uploaded images will be limited to this width (ref. https://codex.www.ads-software.com/Content_Width).
2) Your host must have PHP GD extension installed and enabled, as WordPress uses it to resize images to these specific sizes. If this extension is not active, MailPoet will probably end up using the original image size, which may be larger.
Let us know if that helps.
Best regards,
MailPoet Team