How to set/adjust the alignment of the web page on different screen resolution automatically?

Dear Sir,

I am developing a web project containing the aspx web page. By clicking the pop up link in the aspx page. there is one pop up screen have to open and the user has to select the values in the list. Once the user selected all the required values, there is one submit button to save in the aspx parent page. Unfortunately it is over size of the screen. This is due to screen resolution issue. That pop up page is developed in .ascx. So How to set/adjust the alignment of the web page on different client system screen resolution automatically?.. pls guide me...

siva