Both dashboards have to be same size in order to achieve pixel perfect Email Subscription. If they are different you will run into issue as above.
Yes you are right. When I changed the size of both dashboards to same size - 1000 x 1800. They both readable in the email, but the second one was cut off. I tried changing the size of both dashboards to 1000 x 7000. Both dashboards look very small in the email and there is a big gap between the dashboards. It seems there is a height limit of the dashboard in order to be readable in the email.