Unable to execute the selenium scripts with different browser sizes by default jenkin job is taking 1024X784 browser size but i wont solve my problem.

1. I tried a xvfb it is also not  naking specified size of the brower
2. Selenium code level passing the fixed browser size also tried.
3. Through slave also tried.

Could you please help me on this. i am blocking at this screen resolution fix. Could  you help me should i try in another way to fix it ?