You can either use fill option for browser or page : https://helpx.adobe.com/muse/using/using-fill-browser-fill-options.html
Page width can be decided according to target audience device , with full width contents so that it should fit to screen across different sizes :
https://www.youtube.com/watch?v=XM_8KJzZYMg
Understanding the size and resolution of your graphics | Adobe Muse CC tutorials
Thanks,
Sanjit