Instead of useing px values for widths / heights you can use %.
% values automaticaly resize to a percenrage of the internet browser viewport width/height. Occasionaly the numbers dont add up and problems occur to best tackle these issues float and margin commands can be used in the css.
When useing % values it is best to onely specify widths, the heights will automaticaly perportion to that based on content.
You can use any value you like for % of course over 100 is not reccomended as it resizes to wider / taller than the open window which defeates the point of being able to resize to fit.