Actually if someone help me with removing white space of below design .. as i just have to show one drop down only in interface

This seems to be a record action. In the record designer, you can adjust the size of that pop-up window in 3 sizes

After adjusting the size medium if it looks odd, just use columns layout by maintaining the content in the center column and then try to adjust the value as per your need.

Hi[mention:0591d80bf3a5406ebeb3855605fe90c0:e9ed411860ed4f2ba0265705b8793d05]

As Stefan mentioned, just change dialog box size to small it'll solve the issue.