The Extra Theme includes an optional search box in the secondary header. It is fixed in size and may be too narrow for some sites, especially where you anticipate users searching for long words / phrases. You can increase its size with the following CSS:

/* Set the secondary header search bar width */
.et-top-search .et-search-field { width: 200px !important; }

You can add this to the theme by pasting it into the “Custom CSS” box found in the ePanel at “Extra > Theme Options > General Settings” (at the very bottom).