It sounds like you're trying to create a responsive design for your page. If this is the case there are a few things you should do to help you along this process. There are several really good web resources to help you with this.
There is a lot to digest but it pretty much boils down to two things.
keep your page flexible, all widths in either
%
orem
s. So usewidth:80%
instead ofwidth: 1000px
Use media queries to change your CSS for different screen sizes. In this case something for mobile sized screens.