What you are looking for is something called responsive design.
Unfortunately, Muse does not support responsive design at the moment. However, if you have any experience with web programming, Dreamweaver CS6 support responsive design with Media Queries and Fluid Grid Layouts, which is editing the CSS styling for each screen size.
Take a look at this article!
Hope this helps,
Julia, thank you so much.
Unfortunately, I am new to web programming and have only used Muse so far.
Am I right in understanding that any website designed with Muse will have that problem or does it depend on the layout?
Kati, please also see the following topic and idea:
Europe, Middle East and Africa