Thank you for your prompt response!
While the CSS code has wonderfully done its job, the search box now seems to be left-aligned. Though the height has reduced proportionately from both the top and bottom of the box, the width has cut itself only from the right. This has actually created a blank space on the right in the navigation menu which otherwise is right-aligned.
Can you please help in getting the search box to appear on the extreme right of the menu and occupy a space proportionate to its reduced size?