Hi Sean,
At this time, the in-app editor does not provide options to resize the in-app message container.
When sending in-app messages to a website, you could send them so they’re attached to a container on your site by using the inline positioning option and inputting the container’s `id` value. By doing this, you can use CSS to style the container on your website to the desired size and when the in-app message is received, it should adjust to fit the styled container. Give it a try and see if that works out for you!
Here’s a little more info from our docs: https://customer.io/docs/journeys/send-in-app-message/#display-and-position