Lock website size and dimensions

Is there a cleaver way in which i can have a fixed size for my website.
If you eg look at https://www.aeldreit.dk/vi-tilbyder/ i want to have header and footer showing when people enter the site and at all time. So that the scrolling function happens within the page instead of the on the whole site.

The fixed size has to be the samme for alle my pages within the design.

Regards,

Anders